Given that DH has gone, the fact that there is an OW and he has been messing with the shares is relatively good news
, in that it suggests that the business is not failing and therefore will be a valuable part of the pot of family assets in which you will share.
For finding out about the company, start here:
www.gov.uk/get-information-about-a-company
Important, note the bit that says "You can also set up free email alerts to tell you when a company updates its details (for example, a change of director or address).". So you can track what goes on in the company in future and you should do that.
On the next page, if you know the company name you can input it to see all their publicly filed information. If not - input your name and it will find you and tell you the companies of which you are or were a director. Then follow through to the company information.
If you look for documents called Mortgages and Charges, you should be able to find who (which bank) holds the (second) mortgage on the house and for how much. Even though you may have been a director at the time, when the family home was mortgaged for the purpose of the business, you should have had independent legal advice. I bet you didn't, so that also needs to be raised with your solicitor.
Since the joint asset pool includes a company, you are also going to need an accountant, because valuation of a company can be very subjective. But your shit-hot lawyer will probably recommend someone. As a PP has noted, you need more than a regular family lawyer.
Also as PPs have noted, DH can probably resign you as a director without you signing anything, but you must have signed a Stock Transfer Form to transfer any shares. If he transferred back to himself, that loss of shares shouldn't make any difference to you as they will still be in the pot of family assets. Again, you probably should have had tax advice at the time. Maybe not independent, but you should have received it directly and not via your STBEx DH.