Slitherine

I'm on my first playthrough of this game and I'm loving it.

If I have a province where I have more than 1 of the same building that gives an experience bonus to say missile troops, is the bonus cumulative or are the extra copies of the building in the other regions within the province just wasting building slots?

I believe it's more restrictive then that. Right now, only the buildings in the capitol region of the province (where the troops appear) actually count. I think the dev's have said they're looking at improving this.

Some don't just provide a plus to experience - some provide extra equipment, or infrastructure. Deleting them takes several turns and gives a loyalty penalty. If you get a rebellion you may need them later depending on how your country splits