From experience (Betty's floorpan looked similar back in January) you WILL find a lot more rot once you start chopping back to find solidity.  Regardless of paint removal or wire brushing, once you get the sawzall or grinder in to play you will find very thin metal, and once you start striking arcs to tack the replacement panel/patch in place you will blow holes in places you thought were solid - trust me, I've been there several times.

Undoing the rear leg floor mount won't make things move unduly, assuming your mounts are in good condition.  All my cars are solid mounted and the 'frames just don't move.  If you're still on the rubber front mounts don't even think about it - hell, the front towers can be seen moving when you waggle the steering while stationary.

I tend to jack the car sideways and support it on the rear subframe and front tierod mount - you can almost get it onto the sidewalls for access.

Remove all the lines from under the car, even if you think they're clear.  My main front/rear brake line sprang back in to position due to vibration while cutting and I nicked it with the saw.  Ten minutes more making sure things are clear is worth many hours of sweating to put a new line it and resetting the ****ing PDWA valve...

The subframe is attached to the body in 6 points - two at the front, two at the firewall cross-member (above the springs) and the two extensions under the floor. And then there's at least one engine steady.
With the car well supported on blocking so that the body is supported and the front subframe is supported and shimmed in 4 places, disconnecting the bolts from the right side extension should not result in any significant movement between the floor and the subframe extension. It should just sit there.

You could then cut away the area of the subframe bolts and stiffener plate and replace the metal, providing a new stiffener. New holes could be drilled from below (assuming the car is high enough) using the frame mount holes as a guide. The difficult part might be cleaning up welds and finishing the metal on the underside with the subframe in the way, though once the panel is in place and the holes drilled, the subframe might be loosened and lowered enough without complete removal. But I agree that things would be easier with the subframe and engine pulled completely.

No, I'm certain that metal is fine.  Already polished the paint and surface rust away.  Picture doesn't show that.  If I was to pull the engine and subframe, then for sure I have to support the body somehow.  That would make matters worse and defeat the purpose. 

I would wager that once you start cutting away to get clean edges to weld to, you're going to find more corrosion. It may end up being a moot point whether or not to mess with the subframe mounts. I would just try to support the subframe and the body in as many places as you can, so that once you start cutting, things can't start settling and falling apart. OR, bite the bullet and pull the engine and subframe out. You'll probably be glad you did in the end. 

I've decided to tackle fixing some corrosion on the floor of my 82 mini.  It's mostly at the corners of where the floor, footwell and wheel wells meet.  I've bought replacement panels, and was planning to cut out the area shown with the red dotted lines.  The problem I see is that there is a front subframe mounting point very close by, as shown with the green circles that are the mounting bolts.  I don't want to mess with the mounting point, it is structurally sound and I just need to cut below it.  I currently have the car supported under the front subframe with blocks.  What I don't know is if this mounting point is supporting any body weight or not.  If I cut under it, I'm worried about the remaining metal above the cut buckling.  Would most of the weight be supported at the shock towers?  Does any one have experience cutting like this and do I need to worry about supporting the body in some special way?