valouring up agents

[Knight Templar]

Very good answer, Vlad. Just to add some things:

If I remember correctly, your holy men can also try to offer alliances and cease fires with enemy factions, so that might work too.

You remember correctly. This is the only way to valour up your Bishops, Cardinals, Alims and other holy men.

Spies valour up if they do a succesful 'spy-mission'. If you tell them to go and find secret vices on the enemy king, they valour up if they succeed. (they go from 0 to 1 valour after only one succesful attempt, you'll need more to gain more valour). I guess they also gain valour for catching your own generals for treason, but I've never tried that.

Spies are probably the most complex agents in the game (if you're interested, check this thread https://forums.totalwar.org/vb/showthread.php?t=52284 ). There are 7 missions they can do. They gain valour for four of them: revealing vices of foreign generals, organising a successful trial for your general, opening a castle in a province you're besieging and causing rebellion in enemy provinces.
I'm not sure if they gain valour for informing you about enemy's plans.

How many stars do they have to have before you can use them offensively?

For inquisitors, I don't send them in hard missions (for ex. to burn 7-star enemy ex-prince with good piety) until they reach lvl 5. Grand inquisitors are similar, but they're easier to train as they have more chanes (than ordinaly inquisitors) to successfully burn general.
Training inquisitors isn't hard. Just get your crappy 0-piety generals in a province with high zeal and (grand) inquisitors will valour up quickly. Good thing about GI is that they have good chances for burning low-piety enemy princes at valour 5 (and more).
For spies, I usually never send them in a province with BF unless they are valour 5. You can also train them on your own and/or enemy generals.
For assassins, I tend never to send them in a BF province for longer than 1 turn.