First, I'd stress that RWR isn't specifically known for being a low HW game, but looks like you already tried the demo and if that ran fine enough for you, the current full beta should run fine as well.
1. While map editing is possible using Inkscape, I wouldn't call it a real
mission editor. The mission related stuff you can do with Inkscape is mostly adding and removing the vehicle and soldier spawn points, base locations and optional fixed initial bases for factions.
Soldier counts, initial base ownerships, AI attack / border control / defense force weighting can be controlled on the server with a few XML commands, or through scripting.
So, e.g. you might set up things so that your faction always starts at one specific base, turn off base capturing, set all of the factions to defend only so that you won't risk losing your only base - you need that to exist, and you focus on roaming in the enemy zone, trying to locate their comms truck or cargo truck (or any other vehicle you might want to set up for destroying or delivery), and your faction might have 0 AI if you want to do it purely alone with your coop buddies. You could also turn down the number of bases in the map to have less possibilities where your objective targets are.
We haven't done this in vanilla yet, but it's technically possible to model a static destroy target as a vehicle that doesn't move, and track its destroy-signal to give a reward, or possibly advance in the objective sequence if things are handled through scripting.
Soon, it'll be possible to plant items in soldiers' backpacks using the objective control scripts and track for these items getting delivered to a chosen place in the map, making character / item based missions possible as well.
2/3. Every player can run anywhere within the map, but maps are restricted at 1 square kilometer area. The vanilla maps range from 6-14 bases; the one with 14 bases utilizes most of the whole map area.