Of course, none of us here know the full story but you do seem to be very hands on so I'm sure you'll get to the bottom of it.
The Land Registry staff are exceptionally helpful- they're telling you that they don't know what happened - they can only surmise, until the Instrument(s) are examined by a solicitor and an approach made to them to deal with the issue, whatever it is.
Your second last paragraph is very hard to follow and combined with your tale of woe about the ex-solicitor now acting for the sellers of the house you want to buy, it sounds like a right mess.
As already said, talk to your current solicitor. S/he may simply not have the time to put every other file aside in their office and deal solely with this one. They will work through the process as already outlined.
But if you do want a solicitor to do nothing but deal with this issue until it is resolved, you will pay for the privilege.
mf