As you say, the usless installer didn’t get paid in full, so why not use some to get the work rectified, just to see the end of it. However, make sure he carries out necessary insulation tests prior to upgrading, as you don’t want to run into a rewire, which could be costly.
My last link goes someway in explaing additional requirements of solar PV close to roof edge.
Interesting I’m just looking at a small house (Newbigging Dundee), with what could be a granny annexe thats the middle of no where, which would suit my daughter when left school in next year. Ironically it would have put me in travelling distance to you but, wifey say she doesn’t want Scotland 😂