I'm going to take a stab at this.
It's a very strange scenario.
I would suggest an off grid inverter (located in the new building, to be by the pole), similar to the Sol-Ark.
Wire the house to the regular output.
And the garage to the critical loads/generator connection. (It can output and input, simultaneously)
This is the best option I can see, for your existing underground wiring.
Put solar panels on the garage, with Microinverters.
In the future, you could even add a ground mounted solar array, near the inverter.
The inverter can control the Microinverters, through the existing wiring. And provide grid to all loads, if desired. It could also be set up to start the generator, wirelessly.