Whilst I don't think the front forks on your F700GS have any inbuilt adjustment, they should perform Ok as they are. Some riders play around with progressive springs & different weight fork oil to change the damping characteristics, but I doubt this is actually the issue here. The seat height change is minimal 5/8 in/15mm. I assume you have the standard chassis height, so why do you need to be looking at lowering the ride height? I've seen it done, but frankly, if you are tip toe & uncomfortable you need to think about swopping to an OE low model.
Setting the suspension up correctly is pretty straightforward on these. Looking at your weights & assuming there are no defects with the machine mechanically (tyre pressures/head bearings/wheel bearings/swing arm/front forks level in the top yokes, not dropped above) I suspect the instability issue is with the rear shock. Far from slackening off the preload, I would expect you to need at least 2/3 of the available preload dialled in, with a pillion, as well as an increase in damping. Thus I suggest you take the settings back to factory standard. The tool required is under the nose of the seat. My Rider's manual shows this under 'operations(4) - adjusting spring pre-load, then over the page to the basic damper setting, probably 2 turns out. Then road test the bike solo - all should be well. Then, once you are happy solo, you can get to it & increase the preload to take account of the extra load of the pillion &/or luggage. One way is to sit on the bike solo, headlight on pointing at a wall on dip - mark the wall with say tape at the dip cut-off. Then wind up the preload, add the extra load & see where your beam sits now. Carry on adjusting until the beam cut off is corrected with your full load on board. Then wind up the damping progessively until you are happy with its characteristics, say a couple of turns at a time, then road test.
I hope this helps, indeed have I understood the problem correctly?
Ride Safe ...................... KEN