"We have taken further legal advice and it has been stated that unless the subsale goes through before or on the day of the initial planned closing date between parties A and B, then party A may be liable by Revenue in the future for stamp duty evasion."
It is perfectly possible to structure the transaction in a legitimate way under current law.
"We would also be signing a legal document saying that the subsale went thorugh on the planned closing date as had been aranged as opposed to when it actually took place in the future presumably when party B had found the buyer i.e. party C."
Such a document would indeed be fraudulent, and you should not sign (nor should you be asked to sign) it.
"Furthermore, the advice also suggested that agreeing to this subsale contract may undermine our existing contract, our entitlement to forfeited deposit, any planned future suit for damages and any planned future suit for specific performance re contract enforcement because it has put another condition (that is subsale) onto existing contract. "
To be frank, this sounds like the advice of a lawyer (btw please don't name them) who is happy to point out problems and perhaps not so quick to suggest solutions. None of these issues is incapable of being dealt with in a clear and contractually binding way by agreement between the parties.
At the end of the day, you are not contractually obliged to facilitate a 'pay now, transfer deed later' type of subsale. However, if this is the only way that you are actually going to get to 'pay now' and if it is structured so as to properly avoid (and not evade) stamp duty, then you might consider it worth your while. It depends, I suppose, to some extent on how much stamp duty we are talking about, and whether it is a large enough sum that the buyer might try to dig in their heels about it.