Hi,
First question: I think it is fairly active, as much as any other mod community usually is this amount of time after a game release. Has the community largely been turned off by a lack of urgency to fix broken modding tools, lack of official mod support in multiplayer and some elements being totally moddable while other elements are mysteriously impossible to mod (especially certain art files) - yes, most definitely, but I don't think everyone has given up on it.
One of the biggest problems is a lack though of updated modding material, if you follow guides from the very beginning for say creating Civs etc, you'll end up short of a positive result. You have to dig around, read thoroughly through threads to find updated material, since patches etc have changed a lot of significant things. But people are still produces mods, still answering questions and helping people. I think it will likely pick up again after the expansion pack AND if the .dll code (I think it's called, the core stuff) is released. In other words if they open up the whole game to full modding (which I think they have promised is coming soon-ish) then you'll see a massive resurgence in modding. I think until now most of the community has just been frustrated that the vast majority of modding is work-arounds, and until that changes, until all the tools work as they are meant to and until ALL files are fully able to be ripped apart, put back together and added etc, it'll remain fairly quiet. The Civ 4 modding community seems a bit more active to me, but that is just my opinion. Oh and you can't play custom maps in multiplayer - that is so beyond annoying to me it is untrue, so if you play with friends then don't bother creating a fun map for it (but custom scripts work I think). Just wanted to add that on there.
Your second question; Yes. The mod tools were built to be compartmentalised and modular so that mods could be easily activated alongside one another - are there some conflicts? Sure, probably but as of yet I've not seen any that can't be overcome. It's not Civ 4 that is for sure and that is a good thing.
Your third point: I simply don't know, if you've searched the forums and can't find it then probably not, but that side of modding (map scripting) seems very alive to me - I'm sure it could be done either by yourself with some tutorials or by asking another, perhaps more experienced community member.
Sorry if that post was a bit rambling, I just saw your post and decided I might be able to help - and much of the first question is subjective with many reasons for 'why' and many points of view. But it isn't 'dead' just hibernating. Also, get the game! It is honestly well worth it, I'm a longtime Civ player and I've never understood the tide of negativity toward it - I think much of it is mod community anger, but it stems more from Firaxis over-hyping the 'moddability' of the game as much as anything else. But the game is very sound, especially with the DLC and all the new patches etc it is a very, very full game. Also I still think despite huge problems I prefer the way you mod Civ 5 to Civ 4 (I modded Civ 4 heavily as well), and the in-game mod search and install etc is fantastic. I think it is radically superior to Civ 4, but that is a topic for another thread and another day.
EDIT: I would say this, be positive about the modding of this game - my comment may seem a bit doom and gloom but actually I think it's best modding days are ahead as I said with the release of the DLL etc. If I were you I would get the game, get involved now and you can be in full stride when the DLL comes out, the tools are fixed properly and possibly mods become officially supported in Multiplayer.
