Most people don't play with sideboards in Commander, so it's generally safe to just assume they aren't allowed, but having a bunch of alternate cards to swap in and out is totally fine. I certainly have some weirdly constructed Commander decks, like the one that's only 80 cards and I randomly shuffle 19 more cards into it from a preselected pile before the game starts.

In terms of life gain cards, I have a lifegain deck and it's pretty fun. Cards I've enjoyed in mine include literally every card with Extort (basically all of them are playable, though they're better in multiples), Greed for draw, Phyrexian Reclamation for returning creatures from your graveyard, Rhox Faithmender, Boon Reflection, and Cradle of Vitality (the last 3 are kind of obvious but still insane). I notice you don't have Wall of Reverence; that card is responsible for a lot of the stupidest situations I've gotten into.