Thanks for the kind words.
So you know, there will always be a gap behind a modern floor-mounted, downward-discharge toilet. And it will always be free-standing. Most toilets today are designed to have a gap of between 3/4" to about 1-1/8" behind the toilet when mounted on a rough-in that is exactly as specified (i.e. the rough-in is really 12", not 11.5", which one finds pretty-often).
The toilets that are assembled by connecting a tank to a bowl are now called "close-coupled" toilets because the tank sits on and is securely connected to the base. (Of course, some are a single casting -- "one piece" toilets.) "Close coupled" is used to differentiate them from the previous style, which involved a tank that mounted on the wall a couple of inches above the toilet base and was connected to it with a pipe. Kind of like your Victorian toilet but mounted just above instead of way up on the wall. I actually have one of those wall-mount tanks on a toilet off of our laundry room -- with a date stamp in it from 1927.
Just mentioning this because you won't have to jerry-rig anything if you just put the 12" toilet on the 14" rough-in. And it will be just as stable as if it were put on a rough-in that was exactly 12". However, it will have a gap behind it, which may be an issue or not, depending on how it is facing. For example, in my City apartment, the toilet is a 12" rough-in toilet mounted on a 14" rough-in. So there is a gap behind it of about 2-3/4". However, there is a row of mirrored cabinets above the sink and toilet, that protrude forward, so the extra space really isn't noticeable, and I think the architect thought that if the toilet were slid back, the sitting position actually might seem weird. On the other hand, if the toilet is facing sideways as you enter the room, the larger gap might be more noticeable.
I think you are on the right path with the Watersense skirted toilet with the 14" Unifit. I just didn't want you to be concerned that there was something structurally-improper about mounting a 12" toilet on a 14" rough. It's done all the time, and doesn't make it less stable.