Hi,
Short and end of it:
1. Find attributes for something like the Oxford Library (not sure why this is a national wonder but whatever) and edit that, should be easy enough.
2. You'll need to do more work here I think. You'll need to mod several files including the art defines. You may need do some animation work, not sure, hopefully not. You'll have to look through the units.xml and see what is in there, noting the ranged units. You'll need a range strangth, distance etc and you'll have to make sure the gunpowder unit you edit probably has an 'archer class.' But I haven't looked into it too much for you (that is for you to do!) Since the option is clearly available in this game for a couple of classes of units I'm sure it can be adopted by another. However the art/animations is what might break this. When the gunpowder unit goes to 'shoot' what is the game drawing upon? You'll just have to point it to the 'shooting' animation/art that already exists and hope it works, but i'm sceptical. Also look around, it might have been done as a mod by someone else already, I know others wanted this, thinking it makes more sense. Personally I don't think it does make sense - gunpowder units were terribly inaccurate and useless past a hundred feet or so up until WW1. Archery units however could fire vast distances with great accuracy and they could have a 'curved projection' (over blocking obstacles.) English longbowman could fire something like 300 metres (about 950 feet) accurately. Musketman could only dream of that - they only came about as means to penetrate medieval armour. So don't lose any sleep if you can't get it working

It is definately possible, I just don't know if anyone has done it yet, I imagine it takes some skill, not just a few changes here and there. Usually a request like this, this early into a games modding life is just not going to happen.
3. I have no idea what you want here! Again sounds like an animations change. You can point it to new animations but I doubt it'll be that simple at all, never is, and wouldn't really make sense.
Perhaps not what you are looking for but wa are all getting frustrated/stumbling blindly through this, this early on. Point of interest best time to come to a game for modding? About 18 months after release apparently, for some reason, most has been done but people are still interested, and there is often a renewed interest after a lull.
Anyway try it out, keep posting back and let me know how it goes. If it works then you are probably the first to do it (check around though before going it alone) and at least you'll have bragging rights and can tell others how you did it.
Cheers.