Well, I've given pointers here and there but I'm not certain how exactly to teach someone to mod, I've been doing it for so long between this game, RON, and all the Age of Empires games that I just kinda have a "feel" for it, obviously every game is unique, but mostly all I can say is I just dive in and believe me I make tons of mistakes, have tons of failures, but when one thing fails I try another way and another till I eventually get it.
Many things just become routine since you will do them so many times. I would guess the best way would be if you have something specific in mind I could get you started on it, and I know that looking at the .xml files it seems overwhelming, but like I say, after you've done a few things it gets to be memorization and those files "shrink" quite a bit.
The mod guides are an OK place to start however unlike RON there is not a full list of script commands and even the one's they do give have very poor descriptions of what they do, so I really had to do a lot of guess work to figure them out, and some I still don't know which is why it would be great to talk to some of the games designers. I tried contacting the names you listed, so far no replies.
As for mapmaking, sadly that is a pain in ROL, not sure why they didn't have a random generator like they did in RON but if you want to make a map you literally have to build it from scratch, which is likely why there are so few fan made maps out there for this game.
If you want to get started I would say try to enable Venza, she is one of the easier heroes to enable, or maybe if you want to start even smaller you could just try to enable a unit or building.