Enemies do reappear as you back track but they end up being a higher level and very difficult to beat (on difficulty 3 anyway, or even balanced depending on you level) unless you are coming back later in the game. Experience isn't an issue in this game as long as you aren't trying to over level like a JRPG, in fact even after maxing out dozens of runics, all skills and what not I was bathing in surplus EXP from mid to late game, the level locking being the barrier and not the ammount of EXP. It's hack silver and mats you need to be weary of running out of.
I just finished the game on Sunday, and I did a fair bit of exploration. It probably took me 50 or 60 hours to beat. I don't think the enemies respawn in this game. There are certain areas where enemies will reappear, but I think those places are manually scripted to re-appear there. Once killed a second time, they are gone for good. I never found a place where I could consistently grind, but nor would I have wanted to. By the end of the game I had a quarter million XP saved up and a half million hacksilver. Is XP good for anything besides skills and runic abilities?
Nah, not that I recall but there are a fuck ton of runic abilities to be found and Atreus summons which you can feed exp into. It's pretty insane the amount of combat skills that are hidden in this game or tied to side content. One of the reasons I made a thread stating this games side content is up there with the wither 3's as EVERYTHING ties back to the main game and is ever so rewarding.
And yeah, the enemies on back tracking a presets. I think they made the right call not allowing that kind of JRPG over level grinding, it's pretty perfectly balanced between the difficulties although I haven't tried the easiest one and if you are playing on the higher difficulties grabbing those summons, talismans and runics, taking close notice to their substats and switching them out for certain encounters is a must.
What difficulty did ya beat the game on at 50ish hours? And are you going to run through the end game stuff?
Everything in the above reply is my opinion, from my own perspective and not representative of reality outside of my own head!
-Android user, please be gentle with critique on my spelling.