I watched that GWT video and was surprised he never mentioned lifting the mower when mowing the 3ft stuff with it. Honestly, considering that mowing any 3ft grass is going to look like a choppy sloppy mess afterwards whether you do it with a bush hog or a finish mower, the main difference is really the implement's ability to survive hitting the things you can't see in the tall grass. If one were to mow stuff that tall with the finish mower, I would probably lift it a foot or foot and a half on the first pass, and only put it on the ground when I could actually SEE what's on the ground. To me that's the only real caveat of mowing really tall grass with a finish mower, that it should be done in more than one pass, with patience.
Also be aware that you probably won’t be able to lift the RFM with PTO engaged as the tilt in the floating linkages will cause the blades to contact the ground.
I'm not sure what you're getting at there. If no part of the blades extends beyond the front of the deck, beyond the rear of the deck, or below the bottom of the deck, it's impossible for the blades to touch the ground before the deck itself contacts and stops the whole thing going any lower. Also, with wheels at all 4 corners extending the 'pivot points' further out from the actual deck, almost any tilting at all would RAISE all the blades away from the ground.
I think the only issue with raising the RFM with pto engaged has to do with the top link. If your top link is not parallel to your lower arms, it will tilt the implement as it goes up and down, which will make your u-joints slightly unhappy and COULD put you in a bind if your driveshaft is not able to compress/collapse by as much as the implement moves closer to the tractor. If the implement has some kind of non-rigid link to let the deck follow the ground contours, you would want to make sure it can't extend so far that it lets the rear of the implement droop severely when lifted as it would again make the u-joints unhappy and might let the driveshaft come apart if it can't extend that far.
I don't have much experience with RFMs but i have a 48" one I've run on my Kubota B6100/7100s and I've lifted it almost all the way up with the pto engaged without issue.