Sounds very stressful OP. What have the EA said about the sale, I would assume they would not have taken the instruction without seller a & b signing the contract?
We had a difficult purchase 20years ago, couple in the chain were divorcing and the husband refused to exchange etc, all got very stressy, eventually he did and the sale completed. Our last purchase was probate sale, thankfully the family had applied for probate before putting the property on the market, and obviously no exchange can happen before probate is granted.
Is the property currently tenanted? Is that a concern that it won’t be sold with vacant possession? What has the vendors put on the form to their solicitors?
we have bought and sold many times over the years, advice I would give is not to communicate with the vendors directly, buyers always seem to want to do this but it often leads to conflict, better to leave it to the solicitors, but keep in regular contact with your solicitor.
sounds to me that the vendors, know you want the house so they are using pulling out in the hope you don’t want to lose the property etc. should you decide to pull out you will lose solicitors costs so far, and survey etc.
Every time we have moved it’s been stressful, so I can empathise OP. Good luck with whatever you decide to do, but if you really want the house, you need to take the emotion out of the situation, calm things down.
Once you are in the property you will never have to see or deal with them again. It’s purely transactional.