Hi starting.
I am so sorry to hear that once again he has behaved like an utter prick.
Please don't panic though. Remember that his solicitor will basically do whatever twunt instructs him to do - it doesn't mean he's got a case. It's no skin off the solicitor's nose to send out claims like this, knowing they have no basis in reality, because the solicitor will get paid, regardless of outcome.
Him claiming that you promised him X means jack shit - what counts is what's on paper. If the paper records didn't count, then no one would ever have contracts/deeds etc. All agreements would be verbal.
He may claim that you promised him the world, but you say you didn't - his word against yours. The onus is on him to prove it. He can't claim half your property on a totally unsubstantiated claim, otherwise we would all be able to go around making claims against people who have something we want.
You have evidence about the unfinished state of the house and it's current value.
As for the credit card - the whole point of a main card holder is that they knowingly accept responsibility for the card. The second card holder is entitled to no info regarding the account unless the main holder gives consent and has no liability for it. He is chancing his arm here.
I bet his solicitor has said words to the effect of 'scare the shit out of starting by asking for the moon and she might, out of panic and to make you go away, offer you something'. Don't play into his hands - make the fucker dig deep and pay for his legal 'case'.
So sorry, once again. You truly deserve better.