Hi
Regarding raising Celica's level, there is a good option - ask Nel about it, while allowing her to invite other friends. This allows you to avoid most of the routine. And while Nel is doing this, you have 5-10 minutes for other things.
1. This is a complex topic. Most of the community, to put it mildly, is against classic relationships, unfortunately. Therefore, if such relationships are in the game, there will be few of them and they may appear closer to version 1.0. The fact that they will still be there is hinted at by one scene where Celica and Nel discuss this topic. But this scene was released a couple of years ago and there is no continuation for it yet.
2. If you want our blonde to be a decent girl, then it is better not to develop this parameter. And vice versa.
3. The ending of the game will depend on your actions in the game. I can't say more.
4. Be guided by your logic. If you want Celica to fulfill her dream and become a wonderful mage, and not a girl of easy virtue, then you need to minimize relationships with those who consider certain things for adults to be the correct use of the body. At the same time, the more someone uses your body, the less right you have to own that body. Just like in real life...