I can answer some of these questions, [@Klomster]. The main point is that so long as the deity is receiving worship in some form- be it prayer, monuments and temples dedicated to them, or even sacrifice of mortal creatures in their name- they have a pool of divine magic to draw from, which is obviously larger the more popular they are. So far as I'm aware, you don't specifically need to "spend" discrete points of mana or anything to utilise magic, though precisely how divine magic differs from arcane magic is kind of a point we need to establish between ourselves.

So far as I'm concerned, creation of portals to and from the Upper World, or else use of teleportation for the same effect, should be feasible for any deity. They don't, as far as I know, need to take on "mortal forms" to go to the material plane, since amongst other things, they already have their own bodies to utilise.

With regards to army size, I'd use a relatively loose scale - a small army is several hundred troops, a middling army is several thousand troops, and a large army is tens of thousands of troops. An army of hundreds of thousands of troops would be massive beyond what I think is likely to show up in this game, so that should be a reasonable scale.

With all that said, I like your idea for a Forge God. To me, Ha sounds like a fun guy. Badum tshh... I'll see myself out...