1. Apparently it's possible to avoid mental changes and lewdness, it's called a "pure" run/playthrough and the NG+ room tracks a separate score for such playthroughs. Corruption though accumulates passively, and from a few sources like removing the collar and killing demons.
2. Mainly good time management, avoiding removing the collar and using Blessed Waters from the church. I haven't tried it, but it seems to be doable in Normal difficulty. Much harder than a lewd Normal run though.
3. Not an issue as long as you remove the barrier in the demonic domain in Mountainreach. There's a trick to it that leaves players stumped, check the wiki if you get stuck. After that, you can just use Star Knightess in the fight for an easy win.