I would get a new US accountant, though this may not be what you want to hear. You are asking some pretty basic questions and your accountant's responses show that s/he is not well versed in US taxes for persons abroad. It would call me to question what is being filed. In fact, I think s/he is going to cost you.
The joint filing was a bad call. Because you are not a US citizen or resident, you would have had to file an election in order to file jointly. The election is binding and has to be officially revoked if you now want to file separately. The problem with the revocation is it means you can never again file jointly with your husband (even when it may be advantageous to do so). You can google the 6013g election and election revocation for details. There will certainly be articles out there on the pitfalls.
So because you elected to file jointly, your income will now reported and possibly taxed in the US, even though you have no ties there. Again, unless you revoke (which has its own consequences). To be clear, your now having to potentially pay US tax is not the fault of your husband's birth but this election. If your husband had just filed separately, this wouldn't have been a problem and he probably gained nothing by filing jointly with you all these years. See above, firing your accountant.
Point two for firing the accountant, your obligation to paying SS is in the UK and then you are exempt from paying SS in the US under the US/UK Totalisation Agreement. You can google US/UK Totalisation Agreement and read which country has the right to tax you on the SSA website. It is laid out in a nice grid.
Finally, if your last year's refund is being held, you need to call the IRS. They will be able to give you an explanation over the phone. Do you normally use your UK address on your tax return because you should have received some sort of notice? What do you mean when you say they will either request more infor or issue the refund? The IRS doesn't work this way and if they hold the refund there has to be a set reason for doing so. You haven't really given enough info for me to properly advise. Also, why is your accountant not helping here? It is what you pay them for. And yes, you do have a lot of recourse but I need more info on the official reason/notice.