Whoa!!! This looks like a great and controversial posting!

1. Marius was apolitical, and Grandpa G was somewhat neutral but knew how to survive no matter which faction was in power. He hated Napoleon, and disowned his son-in-law (Marius' Dad), so Marius grew up not knowing a thing about Daddy. That changed, and Marius went into hero-worship mode (for Daddy), and even absorbing Daddy's Bonapartist beliefs at a time when it was all irrelevant (Napoleon already died). The hilarious part was that Marius was so ignorant that he didn't even know who the current King was, and shouted "Down with Louis XVIII, that pig!" when Louis was also dead.
2. No argument there. He was a creepy stalker and Valjean was right to be annoyed and suspected him of being a police spy. Marius wasn't raised by wolves. He should have known better and behaved like any well-brought up young man. Introduce himself to the girl's guardian and ASK to speak to her!
3. The Letter under the Rock. LOL. I fail to see how Cosette even got through reading it. Most of it didn't even sound like it was about her, just some vague metaphysical musings. Dude can't write a love letter worth a damn.
4. I think she was 15. And she was also attracting the attention of that handsome lancer, Theodule Gillenormand. I believe in those times, young men could court a girl (in her home, and with permission from her guardians) and eventually a marriage could be planned out in a few years.
5. I don't get the reference? What happened here?
6. That bothered the F out of me too. Marius was being a fool, so obsessed with his debt to Thenn that he delayed in helping "his" girl's guardian when the older man was trapped in a tenement by thugs. And throwing all that money at Thenn in the end, so he can ruin the lives of black people in America by being a slave trader. Knowing that Victor Hugo was anti-slavery, I never quite understood why Thenn would have such a happy ending... has a good chunk of change, and maybe 30 years of prosperity on the backs of enslaved people. He should have drowned on the voyage to America, or gotten killed in a slave revolt.
7. What annoys me is that Marius let his imagination get the best of him, but we have to admit that Valjean never tried to clarify the situation. He just bowed to Marius' authority as Cosette's hubby and allowed them to ghost him.
8. How wonderful it was that Valjean reconciled with Marius and Cosette, 30 seconds before dying! And he could have been happier for MONTHS and part of Cosette's life if Marius wasn't such a self-righteous d-bag! Valjean stole bread decades ago. Marius was technically a traitor, ready to blow himself and a boatload of soldiers and fellow rebels with dynamite for a cause that he didn't even believe in!

Other musings...

Maybe Cosette would have been better off married to Theodule.

You're right about marriage being a "transfer of property" in those times, so whatever Marius said, she pretty much had to do. Victor Hugo tried to make them ghosting Valjean as palatable as possible... "we mustn't blame these children. They were young and deeply in love" etc.

Marius even offered to use Grandpa G's influence to GET VALJEAN A PARDON. That almost redeemed him! But Valjean turned it down, thinking it was better if society thought he was dead, etc. and once an ember of suspicion (on Marius' part) went out of control, Valjean was ghosted and more miserable than ever. SHOULD HAVE ACCEPTED THE OFFER OF A PARDON! Can you imagine how happy all their lives could have been in Valjean's final months?

edit: clarified son-in-law and not son.