There is no one-best-recipe.
Your vassals should never have too many titles - in this case they will have power to singlehandedly overthrow you and usurp your title or make you to lower crown authority or taxes or levy size. And be sure - there will be time when they want it.
Also don't give titles to someone who hates you - in this case make them your vassal's vassal - this is a suitable solution for unlanded sons or brothers. It also makes them harder to raise war for your crown, because they should firstly became duchies, and this involves war with their liege, whom they may hate no so strongly.
Give titles to ppl who have Content trait (+50 relations), to someone with same sins and virtues as you, also try to make your king as good as your can - virtuous, kind, just, prestigious, pious.
Look for vassals' background - don't give land title to priests and mayors - they will have "Wrong government type" penalty.
Respect - is everything. You can fight with rebel vasslas, but not with all of them at the same time.
Hope they will be helpful!
As far as I'm aware, the tax income and levy size you get from any vassal are based on their holdings, and modified by your realm's taxation/levy laws and the vassal's opinion of you (for taxes at least - I'm unsure whether opinion influences levy size). Catholic Bishops have the added complication of paying their tax to either you or the Pope, depending on who they have the higher opinion of. I don't think a vassal's statistics influence any of these things.
Pretty sure any male of your court can be appointed to the role of Court Chaplain - on appointment the character's portrait will change to show them wearing religious garb.
You're right that making unlanded son's Bishops counts as giving them land. Be careful with this though, as once made a Bishop your son is not eligible to inherit other titles from you in the usual way. If your main heir has an "unfortunate accident" you may find yourself suddenly lacking an eligible a successor!
For realms smaller than Kingdom, any lord can appoint a successor for any Bishop that is his direct vassal. If no appointment is made, the new Bishop is appointed internally by the Church (or Pope). Larger Catholic realms must opt for one of either Free or Papal Investiture - Free operates as before but angers the Pope, while Papal gives the Pope sole control of Bishop appointments in your Kingdom and grants you the Pope's favour (useful when seeking to excommunicate rivals, divorce wives and so on).
Best Answer
It's strange that you can connect to them but they cannot connect to you when using Hamachi, since by definition you can reach each other via LAN.
I would like to help but this sounds like a case of Paradox being incapable of writing decent Netcode. Every paradox game has had bad Netcode and been unstable, had connection issues, etc. They claim to have spent a lot of focus on the EU4 netcode and its still not reliable.
Basically, I suggest you just connect to your friend, since the problem may well not be on your end. The mysteries behind paradox multiplayer don't tend to get solved.