Well Grantor, you seem pretty set on doing it all in one image. I agree that the dpi doesn't matter, until you print it or send it to a device that has physical dimentions to it then dpi is irrelevant.

My expectation is that your going to find that trying to get exactly the right amount of erosion or whatever other process with whatever app you use will take a long time and probably need to be repeated with many attempts before you get it the way you want it. That's a lot of waiting.

IMO, go for tiles and use a script to repeat the procedure over many tiles after you got one working right.

I understand about the licensing issues. There have been cases where if you create original artwork based on a map then it might be valid to post it but if your using scans of existing art then it wouldn't. Its a thin line which does bother me a lot which is why I am happy to help out in our CWBP - the cartographers guild community world building project of which all the maps here are CC licenced.

Obviously there is more general interest in maps like Runequest, Greyhawk and Middle Earth so you get more traction with them.

Of all the things you need to build a really big map you need a lot of determination to complete it and a tolerance for technical pain.