@PotomacBobI would relive my teen years again with great pleasure anytime. With groundhog day-ing every day up to, say, five times, just to experience all kinds of stupid shit. No time else I was so acutely regretful of only 24 hour a day than in my high school years (no, tight and missed deadlines at work don't cut even close).
Not that it was all rainbows and unicorns, but it was a great, intense, and interesting time, and just watching it in first person again, even if I couldn't change anything at all, I would enjoy tremendously.
Change I could seek, would, and likely prepare for -- what in effect could effect change on its own -- goes a bit later.
Funny, I sometimes "prepare" for a do-over in my mind, catch myself of taking notes, or even do it intentionally like scanning science news for things a time traveler could stole for great effect. Or simply make a mental remark like, "oh, that's... interesting... in the wrong way. Next time I would try to..." Perhaps that comes with territory, being a gamer at times prone to savescumming and cheating. In no other game than life itself it could be more rewarding (and more disastrous).
I do believe in butterfly effect, and the very fact of my memories of possible future entering the world would likely disturb the quantum state of that universe and reroll all true random elements within minutes (propagating at lightspeed) even before any of my unavoidably different decisions would enact tangible changes, even if it may take days or even years to manifest in macroscopic events differing significantly due to accumulation of difference.
For that reason alone I would not be keen to venture before 1991 or even 1992 if it can be avoided (dissolution of USSR the way it happened was as insanely improbable event as was it unavoidable, and being inside those events I wouldn't be very willing to visit worlds it went badly wrong), and my thirteen... end of 1989... is cutting it awfully close, without too obvious a payoff. (Like, in my individual situation, there's not even a lottery I could win millions a way it would make real difference -- hyperinflation and all that -- while the actual, very real opportunities that time entails were hardly available for a 13-15 year old who's more interested in watching neighbor girls skinny dipping all day long.) Even though it would be extremely interesting to revisit anyway.
On the universe branching, the easiest to reconcile the apparent time travel is that do-over is pure information transfer -- or insertion: there is no technical difference if the do-over character just had the "first life" as a particularly vivid, potentially clairvoyant dream and just woke up next day. (Yes, that's technically "discarding" the "first-life" universe.)
In fact, having clairvoyant dream better explain depply detailed future knowledge that actual time travel with couldn't possibly have world that stay fixed to that extent. The belief that it was actually lived experience is only important for confidence it has significance, and to explain the suddenly acquired practical knowledge.
For the morality discussion, the one's "morality" is always fine tuned to serve one's selfishness. It is impossible to not do harm. Any decision or indecision, the best or worst decision alike will always hurt someone, somewhere, it's inescapable. That doesn't absolve one from investing in the best decision possible with minimizes suffering to one's reasonable knowledge and limitations. But different decisions will result in different harm and suffering far more likely, and choice between those is the "moral" choice that is necessary selfish reflection on one's irrational beliefs and tribalism. Those should anyway be made and proved for one's conscience to survive (and not drive the owner to suicide), but pretending there's nothing selfish in moralistic calculus is... on itself selfish. (Maybe it's just me being accurately aware people mostly inhabit self invented words.)