Homeowners would cover personal property in the garage, but there are several exclusions in a Homeowners Policy for "property not covered" Some items excluded are autos and any motorized vehicle, unless it is to serve the insured premises, such as a lawn mower or snowblower. So, autos, motorcycles, ATV's, personal watercraft, are not covered. There is some incidental coverage for some trailers.
As for Uninsured/Underinsured Motorist coverage on an Umbrella. This is typically available, but this only applies to Bodily Injury claims. It would never provide coverage for your own personal property that is damaged. That is what your own Homeowners & Auto physical damage coverages are for.
I think the real problem the OP is facing is with his own Homeowners Insurer. The only thing he should be out with them on his Home repairs is the deductible on his HO policy. The Home insurer will be in line to try to collect their share of the $20,000 of insurance the responsible driver has, that's where OP may need some legal assistance, to make sure he's first in line with the subrogation process. I'd call the state insurance commissioner before I'd pay an attorney.
If a claim were made on the agent's Error's & Omissions policy now they would likely wait til the dust settled between the OP and his Homeowners Company. If OP can collect the full $20,000 for his auto from the other party's insurer, the E&O policy won't pay anything.
Good luck to the OP.