1. The will won't necessarily tell you about other assets but, as Dzug said, the executor is responsible for dealing with the estate for the benefit of all beneficiaries.
2. Be firm & definite with Welcome. Keep a note of the name of anyone you see or speak to on the phone, & details of what was said, & the date & time. This would be useful if you had to get into a complaint process with them. If they maintain this is a joint loan then insist they produce the original agreement & compare your signature on it (which will be a forgery) with your proper one. If they still say it was signed by you, then a hand writing expert will have to examine it.
3. I agree with you - probate should be obtained. If the estate is simple & the house the only asset then it should not cost anything like the solicitor's quote. It would even be possible for the executor to do it herself, perhaps with some guidance from the local CAB or other free advice agency. The most difficult bit is the tax form for inheritance tax but as it appears the estate (after debts are paid) is going to be well under the IHT limit of �300,000 this should not be too problematic.
4. If you do go down the probate route & selling yourself, you will need to make sure Abbey & the other secured lenders know what is happening so as to minimise the risk of a premature repossession. They won't wait all that long if the mortgage is not being paid, so getting probate is urgent.
Please go on posting if you need more advice.