it is probably a bit late but I would never do a repair the way you have been advised. It is cumbersome and awkward to do.
If you are committed, then I suggest that you use some form of temporary attachment such as self tapping screws or pop rivets to keep the two layers as tight together as possible then tack all around the repair, on both edges. Dress the repair so as to get it as even as possible then fully weld up.
Depends how the original was attached. If it was spot welded I'd plug weld in a similar manner.
For a seam weld that you'd like to overlap I can't think of any reason to weld both sides. Weld from the top (easier), then hammer the joint flat if it's opened and use seam sealer on the reverse.
The Mini floor is originally one big piece from the boot to the engine bulk head. I'm basically cutting out a square to put a replacement floor pan in place.
When welding the floor pan in, as an overlap, would I weld on the edge or slightly in? What's the best seam sealer to use?