An ok film, but some serious problems

1. I think you half answered it when you said "at least in my eyes". I feel the way they did it was the best solution, in my eyes. The director is never going to satisfy everyone. Also, the director probably just felt they should stay true to the original book. The constant use of the words saumensch and saukerl as terms of endearment wouldn't really translate into English, and it just continues to remind us of their place and time. This is how I'd justify the director's and author's decision anyway, and if they'd chosen a different route I would be able to justify that as well. It's just the strategy they chose.

2. I thought it was quite believable. True, only her hairstyle changed, but I thought it did make her look older and older. Again, that's just "in my eyes".

3. This was a film made mainly for a western audience. As long as the actors were able to speak in a convincing accent, it shouldn't really be a problem. We have British actors playing Americans all the time, and Americans playing Germans, Americans playing Brits, Americans playing Australians. They're actors, it's their jobs to play a character that is not themselves. Geoffrey Rush isn't really an accordionist, or a painter, or German.

1. They are speaking English because it is made by English-speaking people for an English-speaking audience based on a book written in English. That's pretty clear. Only in your eyes are films diminished because of this. They speak with an accent because they are German, so they would have a German accent.
2. Yeah well it's pretty hard to portray. Suspend your disbelief a little.
3. I don't know either but it's rare to see nowadays anyway. Even American characters are often played by other nationalities, most often Australian and English.

I agree that saying the odd word in German was distracting.

"How do we make this seem authentic?" "Erm accented English then throw in the odd dumkopf, should covwr all bases"

For number 2. Are you talking about her whole style or just the fact that only her hair changes? I think it was done well in that she looked more mature. Think about when the movie started and she just had short frizzy hair. It looks like by the end she's giving more care to how she looks. Also remember that in WW2 it may have been hard to come across new clothing. Might be she is altering her own clothes so they still fit her. I think they showed her aging very well and I much preferred it over, say, having the actress change half way through.

I can't agree more. You said it all. Otherwise, it was a great movie.

1. I thought it made it more authentic; it immersed us more in the German culture than if they spoke plain English with no German words or accent. If that was the case I would of wondered if I was in England in WWII instead of Germany during long stretches of the movie.

2. Somewhat true...but she could have just been on the edge of puberty without developing obvious post puberty characteristics. She's a slim build to begin with and will probably look like Taylor Swift does now in ten years. You can tell Taylor Swift is an adult woman these days, but back when she was 12-15, if you didn't know her age, you wouldn't know if she was post puberty or beginning it. Plenty of females are the same way. Look at Miley Cyrus these days: put her in some baggy clothes and she looks like a 11 year old boy.

3. Because it doesn't matter as they played Germans convincingly.

My big problem with it the Death narration. When you're going to have Death telling a story, there has to be a reason for it. why is death bothered? what did this girl do that was more important than anyone else in all of history? I understand that the reality of war is an important theme here, but why this girl over all the other boys and girls that lived in those times? If this was told by a human, who had a direct connection, it wouldnt be an issue... but Death as a character has access to every soul that has ever lived... so we need to know what makes this story unique and special, compared to another girl in the next town. Or any girl from all of history.

And she just didnt do much, or witness anything or affect anyone much, or even had an overly tragic life compared the millions of other people who suffered at that time.

Any English speaker has an accent marking their speech. There's no real "neutral." If they don't speak in a German accent, in what accent should they speak? As neutral an American accent as possible? Leaving them open to complaints from the rest of the world that the Germans speak with a US accent? A BBC "World Service" accent, maybe, so they all sound British? Perhaps a mix of accents where each actor uses their native accent, where they clash with each other? Any accent choice will bring in distractions. I think German is the least jarring. Better, at least, than Mississippi or Mumbai.