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).
First and Foremost, you should stop playing as a count, it is no longer as straightforward as it used to be. By playing as a weak starting character, you are intentionally handicapping yourself for a game you were completely unprepared for. Try playing as a french duke to begin with. That means that you will have the power of your liege for defensive wars, and can declare war on people much weaker than you, ie rebellious counts and weaker dukes. You should start out only going to war with people who you outnumber by 50 percent, minimum, since you have to deal with seiging and defended bonuses.
You should try not to rely on warfare. Making the right marriage matches and waiting up to an hour for them to come to fruition is part of the game, (playing the long game is part of all paradox games really). In fact, Marriage is actually a much stronger mechanic for taking territory than combat. Not only that, but marriage is the most powerful mechanic in combat as well, because it allows you to call allies with vastly larger militaries than you can command early in the game. An alliance with the king of NearbyNation will let you call in thousands or tens of thousands of additional troops.
Still, the overall point is to stop playing as weak characters. Counts and the occasional 1 province duke in ireland are not advisable start locations for someone unfamiliar with paradox games, because like real life, paradox combat is very swingy. A 20 percent difference in starting numbers can be a 200 percent difference in casualties. That means when starting out, make sure you absolutely have a substantial numerical advantage against whoever you declare war on.
As for your example of losing a battle where you had superior numbers, you probably made a mistake with regards to combat morale, but its difficult to know without screenshots. Even if they had a vastly better general you should not lose a 1000 to 2500 fight, unless you had virtually no morale (probably from not paying the men).
Basic Concepts to Practice:
Revoking a county title as a Duke. This usually starts a war of rebellion that you can crush.
Invading an independent county as a Duke. This includes ones rebelling from their liege that have become independent temporarily
Marrying daughters into a powerful alliance (nearby, but not where you want to expand to)
Marrying sons to inherit land
Keeping your most important vassals and advisors (particularly the spymaster) happy.
Best Answer
No. The only ways to gain threat is by expanding (gaining counties) or having your vassals expanding. Raiding ironically doesn't raise your threat.