Issue One - there are two charges to be removed or two Solrs undertakings provided to permit removal of both liens post closing and Issue two - they don't know who will sign the deed of sale, these are causing the delays.
These issues are one and the same really.
If the Vendor signs the deed as mortgagee in possession charges and burdens registered after the charge can be extinguished. However if the Vendor signs as Receiver this is not the case so those burdens have to be dealt with. Either there is sufficient money to pay both the charge and the burdens, in which case no problem, or there is not sufficient, in which case the charge holder has to negotiate with the owners of the burdens about how much they will accept to take their burden off the title so it can be sold.
Your solicitor is well aware of all this, I am sure, and will be pressing the solicitor for the Vendor to give answers.
As a matter of interest, what did the contract say about who would execute the deed?