1) English archers can only plant stakes during the deployment before the battle, not once the battle begins.

2) You can't, directly. All you can do is get their piety up and hope for or create vacancies in the College of Cardinals. Get a Theologians guild if possible, and build a Cathedral in the city where it's located. Train your priests there. That'll give you a bonus to initial piety. Put them in regions that aren't mostly catholic and/or that have heresy present. Let them convert those over to catholic, which will raise their piety. Use them to hunt down heretics and witches.

3) Oh, yes, they most certainly can. A unit won't cause friendly fire to itself, but it will cause friendly fire to others who happen to be hit by the arrows.

4) Assuming you're referring to casualty figures, 42(23) means the unit took 42 casualties during the battle, 23 of which were due to friendly fire.

5) You can't get other factions to GIVE you regions, you always have to make the request yourself and offer something worthwhile in return, whether that is money, another region, or a much needed peace agreement.

6) Mercenaries are quite helpful, but not all units are good. Use mercenaries to either bulk up an army when you need more troops right now, or to fill gaps in your order of battle. For example, England lacks a decently armored spear unit. Hire mercenary spearmen for that purpose.

7) Use generals to lead armies whenever possible. A general with the right traits will make a city run better, but over time generals sitting around in cities tend to pick up a lot of bad traits.

8) I had one merchant making 840 florins a turn sitting on a gold resource in Timbuktu. Others have had higher. Taking out another merchant can get you several thousand florins instantly. Yes, I'd say they are useful.

9) Captures only happen to routed units. Anyone you'd kill while chasing them down is taken prisoner instead.

10) It can change the weather. Bows and gunpowder weapons don't do well in the rain. Trying to control a massive battle in fog or blizzard is much harder than in the clear.