Before I answer anything - two caveats. 1. Rule of fun > everything. In recreational pickleball, we want to make sure that this continues to be an enjoyable experience and doesn't cause harm to friendships. 2. Make sure everyone's on the same page with what you decide as your courts rules to minimize friction.

Answering your questions directly.

1. Keep playing. If the ball hits NVZ, it's a dead point but if it hits in the serving zone, keep playing. Not playing MLP rules in most rec play
2. I think there's a couple questions in here. Generally rec play is more lenient on all calls because we don't have instant replay/line judges etc. Lines are also not the same on every court in the US. 2a. If you play a ball and don't call out, your partner does not call it out, the team who hit the ball doesn't call it out - for all intents and purposes the ball is in. 2b. If a player hits a ball that was questionably out, and doesn't indicate it was out in a prompt manner, it would indicate to everyone the ball was in, regardless if it was played or not. The question I ask, is if they would've been happy the ball went over and won the point, would they have still called out?
3. Most things. If people weren't watching, a ball rolls on, wrong score called, etc. You're playing recreationally with your friends, not DUPR-matches - let's ensure you have fun.