A few questions

[caravel]

What dates do gunpowder and the compass become available? And are there any other events that can be used in "27 Invention requirements for building Label" in the build_prod?

Compass is something like 1170 and Gunpowder is the mid to late 1200s sometime (sorry can't remember the exact date).

Unfortunately Gunpowder and Compass are the only two hardcoded events. The compass event is actually pointless as all of the ships requiring shipyards that depend upon it are high era (1205) anyway. If you want to give the compass event some meaning, then set those ships to all periods availability you should be able to build those ships in 1170.

And also, is it possible in some way to make buildings not get sacked and destroyed when you take a province? This severely hurts the AI's teching ability since whenever they lose their provinces they go back 20 some years of buildings..

Unfortunately that can't be done (how I wish it could). The only non destructible building is the land clearance from VI (in fact I think there is one other from VI but I can't remember it offhand). That one is hardcoded to be indestructible. So if you want a building that cannot be destroyed, use that one.

And another question, after I start a campaign, is there any way to tell which difficulty I chose? I intended to choose expert but have forgotten.. I'm not sure anymore..

Well there's no way to tell as far as I know. In general with Expert the AI will be very difficult to rout due to the moral bonus they get, so start fighting a few battles and you'll know soon enough.

As this thread discusses mainly modding issues: Moved to the Engineers Guild.