I will try and make it clearer - it is an odd area so I do understand the confusion (indeed the behaviour of your buyers makes it more confusing again.
1. At present the Estate agents have found purchasers who wish to buy the property
2. You have (entirely reasonably) said that you require two weeks between exchange and completion
3. The buyers missed an exchange deadline but are now offering to exchange with completion on 5/9.
4. You have not unreasonably refused this completion date
If the buyers agree to exchange on say Wednesday with completion after 14th September then this would be reasonable and the Estate Agents will have preformed their contract and will need paying whether or not the sale goes ahead if YOU and your husband now pull out of the sale UNLESS it is because a third party offering more.
You wish to own the property and to buy your ex out.
You are willing to offer slightly over what is currently on offer.
1. If you do that whilst the other buyers are still willing to go ahead the Estate Agents fees will still have to be paid so they are wasted - and your ex could reasonable say he will only agree to your higher offer if you pay them
2. If he will not agree (and the past history suggests he wont) then you have to apply to the court to vary the existing order. The court would view your paying the wasted costs as being reasonable if you wish to go ahead
2. IF the buyers do pull out you are in a very strong position and of course no wasted costs will be payable