As to Adam singing on top of the bus, I'm assuming it was just a way of him to practice a future performance without having to put another stop in their journey. I must admit, it looked like fun!
Adam explains that he bought the bus from some Swedish tourists.
We might never know for sure, but I think it's fair to assume that Tick was perhaps bisexual, that he gently loved Marion, but after a while the relationship just fizzled out. Judging by their affection towards each other, there was nothing malicious about their separation.
As for Bill's marriage to Cynthia, it's entirely possible she wouldn't agree to an annulment. After all, she did at least want to stay married to him long enough for a green card, or whatever the equivalent is in Australia.