I'm not very familiar with GMRS, but your scenario is what that license is designed for. So I'd recommend it.

I think in some places you can set up a repeater for GMRS? You'd have to check up with the local regulations.

Of course, you could get everyone (or a few designated people) ham licensed. In many areas, operators are allowed to let other people use their radios under their supervision, so you might not need to even get every single person licensed.

Frequency plays a very big role, and the rest "depend".

There are some frequencies that'd just simply be impossible to use for communication that close (in the grand scheme of things) and others that would struggle to penetrate the trees.

Without knowing the area, I'd guess that 2 or 6 meters would be your best bet. (Probably 2m)

Your next challenge would be getting good antennas. The antenna matters a lot (even more than power output. (See me hitting Australia with 7w))

I'm always available for questions via email. You'll find it in my website.