Welcome and glad to hear you're enjoying the game.
1. You can sell the scrolls you don't need in any castle. You can also drop them at any time, though it's better to sell them and earn at least
some cash. The skill "Trade" from the Mind tree will increase their selling price. As to which spells to scribe, that's too complex a question to give a straightforward answer. It depends on your playing style. Probably the spells to scribe are the ones you've used a couple of times and found very useful.
2. Seems fine to me. Don't really know what to comment on here...
3. Hey, no shame in sneaking around grabbing stuff.
As for the next spirit, you'll be able to get her in the next land, i.e. after you finish the main quest in Freedom Isles (retrieving the maps). You'll also get the last spirit there, but for the first one, you don't have to fight at all, and can just grab her and go back to the Isles with her.
4. Aside from buying troops, spells and artifacts, you'll be able to use your money later on, yes, but I don't wanna say what for. Not that it's a huge secret or anything, but it's better if you find out yourself. Let me just say it's in the same place you'll persuade the last spirit to serve you.