Just sitting here thinking if I could fox this without opening walls up.
I think I could, would be difficult and not 100% certain to work, but I’d spend an hour or two trying.
The valved outlet… potentially you could get a 90 outlet with a 1/2” female connection. You could then wind it on to the valve. The valve/hole would be cover by a chrome echelon.
This obviously depends on being able to turn valve on, if it’s off and finding the correct parts. A quick google shows there are female connection outlets.
Second option would be to undo the valve, pull it out and fit a female iron on to the nut/olive in the wall. Could be possible, potentially using screwdrivers/steel rod inserted through tile hole, down the side of valve/new female iron to hold the nut while undoing/doing up. Might need to enlarge the tile hole a bit, not difficult with a 6mm diamond angle grinder bit to slowly enlarge.
Option 2.5, same as above, get valve out and using a soldering coupling, extend pipe out of tiles and fit suitable outlet. Should be enough pipe to solder to even with nut/olive left in wall. If not, use 15mm pipe, a coupling, then small bit of pipe (2mm longer than slip of above coupling, then slip coupling. Slide the above into hole and slip over pipe in wall. Once on, hit with hammer and hope the olive moves back, until enough room to solder.
The problem with the handles, doesn’t seem to difficult to resolve if you’ve got the tools/ability or are willing to pay. The below would likely be a lot cheaper than opening the walls.
Instead of trying to pack the brass spindle adapter, just extend it.
Either some sort of fitting you could screw on to the end of existing brass (using exsisting/new longer screw). This would be more difficult.
Or add an extension (same diameter as where the handle set screw tightens on existing brass) to the existing brass. This should be hollow to allow the screw to be done up. This extension should be silver soldered on to existing brass.
Could make the extension out of something like a brass 1/2” nipple. I would as I don’t have brass stock.
If you found a brass machine shop, they’d have all 3 done in an hour. My friend makes brass radiator covers etc and could easily do this.
Here’s something similar I made using my mates lathe/milling machine as the apprentice lost one of the originals!