I'm interested in building one for myself but I fear that future patches could remove it and with it its stored content...
I don't understand why patches would remove it? You can't pick up a floor or a door. They're static objects. You should be fine!
Teleport? What do you need to do to get this feature? I absolutely hate fast travel, but game so far hasn't given me any proper gameplay mechanics to avoid it.
I think they mean the mat item for changing your fast travel location in settlements and found In one of the building options (I think under miscellaneous somewhere?). So if you place it on a floating palace or anything, that is were you teleport/fast travel to.
Might depend on the settlement, but I built a tower in Sanctuary that was about 4 or 5 floors high before I hit the outer limit.
My house in Sanctuary is 4 floors high and placing things on the roof (floor 5) turned out to be very quirky. Might be because I used the large pre-fab wooden house as the 3rd and 4th floor.
I did not count the floors, but at Greygarden I was able to get onto the second, higher, highway overpass.
Not sure how you pulled that off considering the construction ceiling is 4 stories.
Just in case you wanted to know
Built a house around the worktable and placed one of the Floor - Stairway pieces, then a flat floor at the top (to give you room to turn to your right) then a Stairway, another flat piece and then turn right for another Stairway and so on all the way up to the lower overpass, where I placed a few horizontal planks to get me onto the overpass and then I found I could extend floor almost to the other side (not quite, though).
On a lark, I continued building my right-turning staircase and I was able to get up to the second overpass. Come to think of it, I think I was able to actually place another stairway floor piece there, which let me climb on top of THAT (although I don't think I could link a floor piece to that top-most Staircase piece).
I share the OPs concern that patches could effect the "floating glitch" many are using to clip / position entire structures to locations they cannot normally be placed. Bethesda is fully within their rights to remove / alter the code to "fix" that glitch and let the structures fall where they may. I doubt even they know what would happen to items placed by the glitch if they somehow alter the code to remove said glitch....
I'm going to avoid using that feature for the time being as I would be devastated to build an elaborate tree house just to have it destroyed by a patch...