There is a modding guide in Docs folder under teh directory you saved DomII in if your DomII installation has been updated. It probably is as otherwise there wouldn't be the Mods -folder or option to use mods at all. It has answers for most of your questions. IIRC ranged weapons use the general weapon tags (normally modifying attack, defence etc.) but with different meanings (precision, range...). This is true of some other commands as well. You can add new pretenders, but I think the only way to choose some but not all of those normally available to nation would be to clear the old ones away. I think it is possible, but haven't had the need to check it myself.
If something is not mentioned, it is not possible to do that. Themes are one of the things that cannot be added. You have to work with multiple mods and replace one of the existing nations.
There is a list of sites, their effects and the level needed to find it on an excel file. I know they existed on Sunraybe's site at
www.freewebs.com/dominions2 but it is suspended until the end of month. Too much traffic... Arryn's page might have them. Someone noticed that some information wasn't correct, but it should list all the names and most information is right.