All these questions would need to be forward to a Structural Engineer or a good Architect.
These are the people who are able to give you proper information about the house after they have viewed it.
They can also help you out with Building Regulation issues.
In our experience it is always best to improve the property and get the proper Completion Certificate in place so you do not have this type of a problem when you sell the house on.
The cash you invest in making any changes will be returned upon the sale of your home.
If you try selling as the current owner is, then you will be faced with the same issues.
A good Architect should be able to retrieve copies from the council for this loft space from the councils archives of plans if the people who converted it applied for Planning Permission.
As far as indemnity insurance goes please refer this question to your insurance providers and state your concerns to them, this should answer your question on this matter without any guess work.
