I am trying to make a 2024 version of my paladin. I can pick Oath of Devotion because it is free. I can pick the Xanathar's options and Oathebreaker because they are from 2014 content I own, but I cannot pick the 2014 version of things I own if they're in 2024 content I don't own. I can't use the Oath of the Ancients.
Yes, you could. The most math you ever have to do is divide by 2, add 3. It's time consuming but it's not difficult. The hardest part (which DDB can help with even if you're NOT using the character sheet) is just searching for lists of valid options (i.e. a list of spells you are eligible for, a list of feats, etc.)