I liked it, but what was the point?

I understand you, I think it's a film that you can take many meanings from.

In one sense I feel it's about grief, and how one tragedy can ripple through peoples lives. Lisa couldn't let go of what happened to the point where everyone around her felt smothered and bonds were breaking.

Another part of me thinks it's about boredom in life. Maybe I chose the wrong word, but I think Lisa was tired of her same routine, and so when this tragedy happened she tried to cling onto it because it made her feel special, or was an outer body experience that she didn't want to let go of. Even with Lisa's mother, she was on stage every day, she even dating the French guy out of habit, and wanted to break up with him.
If you notice we're invited into a lot of repetitive scenes, like Lisa arguing in school or walking home and Lisa's mother appearing on stage repetitively , each time looking more bored and fed up.

Just my theory.

I agree with you especially on points 1 and 4. Much as I am a fan of Jean Reno this sub-story just seemed to be no more than a distraction from what should have been the central thread. The focus really should have remained on Lisa and this was not much more than a time-wasting diversion and one more thing for Lisa and Joan to argue about. In a huge shaggy dog finale, Ramone dies off-screen anyway and Joan and us are right back to where she started, except we know that she's had a few dinner dates and received plenty of flowers.

I definitely think it is intended to be a coming of age story, but I also have to ask where's the growth and development of Lisa's character? She was shouting and prone to screaming at the start of this long film and she pretty much still is, close to the end, apart from the supportive sobs with mother at the opera. One has to ask the question, whether the obviously intelligent Lisa has actually learnt anything at all over the journey we have seen her undergo. I'm really not so sure.

BTW I do think Anna Paquin in the central and largely unsympathetic role was quite outstanding.🐭

To me, the point, most simply or globally put, was loss of innocence. The poem and its protagonist Margaret, so central that it/she becomes the film's title, are mirrored in Lisa who imho makes a similar 'arc' over the film's plotline, fleshing out in considerable detail and vividly what the poem's Margaret experiences as an evolution from child to adult in terms of "coming into her emotions" as they gain truth, meaning, and power from lived experience.

In this case, the pivotal lived experience is the trauma of witnessing death, moreover of feeling complicit in that death (but in a philosophic sense where it's not a huge stretch to see us all as complicit in death (e.g., turning a blind eye to poverty and starvation). Here the event that triggers all this is visceral and the contact is so direct that Lisa is literally covered in the blood of the deceased stranger who she'd suddenly become intimately connected to - one whose death she hadn't willed, was horrified by, and yet she had played a role in causing.

That theme - loss of innocence - is the overarching one, as I see it - with matters of grief and guilt and adolescence and family and more as subthemes that the film is also addressing as part of its point, in interwoven threads of the story. To me, Lisa is a very credible depiction of adolescent lostness in the face of expected, desired, and resisted transitions to adulthood where such things as witnessing trauma and tragedy become tests of one's moral compass, a test we see Lisa flounder through - as with virtually all of her relationships - she's all over the map, so to speak, but in a way that grasps and conveys a typical searching in adolescence, seeking connection, meaning, authenticity, identity (those latter two often at odds with each other), purpose, guidance, some sort of ground beneath her feet.

Because her relationship with her mother was the most in focus of her various depicted relationships, it made sense to me that we see Lisa confronted with observing her mother Joan go through stages of an adult connection that Ramon brings into her life. Plotwise, his introduction of Joan to the world of opera served to bring in a broader level of the universal themes of life and death in the quest for love and connection, sensibilities that Lisa is eventually powerfully impacted by in the final scene. Even when the words of opera may be in another language unknown to the spectator, I know from direct experience that the world's most famous operas have become so precisely because they have succeeded in tapping into emotional connections that bridge across language barriers and touch precisely such themes as loss of innocence, guilt, grief, issues of moral compass.

As Lisa is captivated by the opera singer's powerful emotional expression, what I sensed it tapped into was her yearning to understand what life is supposed to be about and how to manage its tumultuous complex emotions. Opera is almost inherently a story of passionate quest - something that Lisa could hear as validating her turmoil as being indeed what adulthood and life are largely 'about'. And she had seen from a certain emotional distance/resistance (very typical adolescent daughter's defiantly seeking self-determination with the fallout of being emotionally chilly) how her mother had just gone through loss and grief with Ramon's death.

Turning in full open and willing vulnerability to her mother in the end - and the fact that it was Ramon who had caused them to both be there in those opera seats - operatic release became the catalyst for taking down the emotional wall that had built up between them. For Lisa, i think there was a glimmer of significantly evolved recognition of what her mother's incarnation of adulthood did and could mean to her.

As a key sidenote, both Lisa and Joan had the experience of being reminded (by virtue of Ramon's son's role) that none of us ever fully know just what and how much we may mean in another person's life. The film began with such an instance in a tragic case, but it ended with an instance in a healing, restorative sense - and also one of some degree of surprise. (I sensed that Joan was never quite sure what Ramon meant to her and didn't in fact "relate" to him in many ways, yet she came to have feelings for him, and loved him enough to grieve him genuinely. The son's message about how much Ramon saw her as his 'connection' pointed up how life isn't blessed with symmetry but instead with accident and asymmetry and disconnection. But the film seems to say in one of its corollary points that we can find ourselves surprised by the impact we can have and feel across those divides, in both unsettling and settling ways.

I'd like to take one of your questions and try to expand from there.
Your question: "As an audience, we can tell that their (Lisa and Joan's) relationship was rocky and a bit tense, but how was the nature of their relationship relevant to Lisa’s character, development or who she was as a person?"

I think the moment Joan tells Lisa not to rat out the bus driver is key to the entire movie. I watched the film several times and I can tell you before that scene, both get on well. They are each self-absorbed but clearly love each other. Their relationship wasn't rocky or tense in the beginning at all. It was a mildly snarky but typical mom and smart teen relationship.
But the second mom tells Lisa to go easy on the bus driver Lisa begins to withdraw and that's when the rocks and tension begin.
You can see it in Lisa's expression when mom tells her. Someone else here posted Lisa "bought it on autopilot" but soon began to see it differently. I would say Lisa saw it immediately and couldn't believe her mother would tell her to lie. Their first real fight comes shortly thereafter. Lisa, without any practical guidance from adults, then takes the whole accident on herself without sufficient mooring and recreates the tragedy in her own image.

Two scenes are key and I would say form the whole basis of the movie.
The first is Matthew Broderick as the teacher reading Spring And Fall: To A Young Child by Gerard Manley Hopkins, where the film's title, "Magaret," comes from, which, obviously, isn't an accident.
The poem is about a child, Margaret, grieving over leaves falling from trees in fall in anticipation of winter. Margaret will experience deeper tragedies growing older. But the real subject, in each case, will be Margaret herself, and her own mortality.

Skip ahead to the scene with Lisa and Emily, the real friend of the woman run over by the bus. Emily scolds Lisa for turning the accident into something about herself, the spiritual connection Lisa now thinks she made with the dying woman in her last fleeting seconds over confusion over the name Lisa, shared both by Lisa and the dying woman's daughter.
But only Lisa was confused, the woman was dying, which brings on its own kind of confusion.
Emily basically says how dare you turn my real friend into your self-absorbed little melodrama. Lisa has to work through that. The hugging with mom at the very end shows she's beginning to see the broadness of life, not just her own little slice of it. And isn't that what growing up is all about?

I really enjoy Lonergan's other films, but this one was trying too hard to be profound, but never succeeded. It was just overlong with too many superfluous subplots. I'm not sure that including the edited out scenes would have helped.