Just to be clear.. the unit doesn't use 2.4 & 5.8 at the same time..
2.4 is default but if it's congested the GLX will swap to 5.8.
You can set 5.8 to be default if wanted.
I recognise those photos Bungo.
Yes, only one other cab can be added to the combo. Or 2 cabs when the head is detached.
Always use 8ohm cabs and your safe.
Sounds like your doing all the right things to reduce feedback risk.
Outdoor gigs usually mean a hollow stage, that can be a problem too, with vibration causing feedback issues.
Do it.. jam it behind the tailpiece with a bit of foam, that also helps stop it vibrating as a potential cause of feedback. Used a G75 for years, now upgraded to a Shure.
Buy the Platinum Pro. It does exactly what you need. The Jtone pickup is a cheap pickup, there are many options out there which will improve your sound.