Season 1 Finale: Why does she want to go back a second time? I get it that it's not nice to have to deal with your brother's death again, but in this timeline he's lived an extra 10, 15? years, and as far as is shown everything in your life is much better. You're more successful, your sister is not married to a loser, your brother's lived longer, your parents get on better and your dad's the writer of some Spock related best seller. What's your issue? Why go back and set up a time capsule letter?
Because she lost 10, 15 years of her life. Yes, things seem objectively better from the little she saw, but it's still not her life; she didn't live any of it and doesn't remember any of it.
As for the series finale, how is Erica supposed to be Sarah's Doctor if she's not allowed to have contact with Dr. Tom (Sarah's father)? Does it not count if she encounters Dr. Tom from before he became a Doctor? What if Sarah has a regret involving the time she stole from her parents and got sent to jail (after Tom became a Doctor) or the day she ran away (when Dr. Tom jumped back in time to re-experience the day with her—there may also have been similar occasions during Tom's own therapy that just weren't depicted on-screen). The interaction between Future Erica and Doctor Tom indicates that the rules can be bent for times taking place before he leaves, but this isn't confirmed in-series.