[Sombre]

Well you can give them crazy powerful units by simply turning one unit they've built a lot of into a much more powerful one. For example unit number 11111 (let's say they're playing Ulm, you want to turn ulm infantry into black templars or something) gets #copystats 2222222 #copyspr 22222222 or whatever. I'm fairly sure the changes will take place to units that have already been built (I know the sprite will, not sure about stats).

The way to add gem, resource and gold income midgame is to select a magic site you know belongs to the enemy nation (like forges of ulm etc) national site and give it #gems 4 100 #gold 5000 #res 1000 or something. Have a look at national site modding in the modding manual for specifics.

If you want to give them a one-off boost just give them a big income from the site and then remove the lines from the mod one turn later.

You can't give nations flat resource or gold % increases via modding.