1) yes, although there is currently no selection process for active mods like in GC2 so they could interfere with one another if they try to do similar things
2) not sure if you mean on your computer or remote, locally it is in Documents/MyGames/Galciv3/Mods , remotely once 1.1 is live steam workshop will be up for uploading and downloading them, i know it is live with the opt in however they mentioned things could possibly be wiped when the patch goes live, i personally have not seen the workshop atm myself
hopefully helpful
edit: this thread https://forums.galciv3.com/465803/page/1/#3569694 he is trying to make a list of ones available atm in the forums but i imagine the workshop will be where you want to go once it's live