Splitting 150amp Service (midwire) from home at 100amps
You're not going to be able to avoid touching the existing panel.

The closest you can come is installing a new main panel where you cut your feeder. Then power the existing main panel from the new panel; your existing panel becomes a subpanel. However, you will need to remove the neutral/ground bonding from the old panel, and depending on how your ground rod is connected to the old panel you will have to move it to the new panel. And because you've now touched the old panel, it likely can not be excluded from permitting and inspection, and if it's as bad as you imply, then you'll have to fix it.

And since you're now in a position where you have to fix your old panel, forget about fixing the old panel. Instead, replace it. And run an 80 amp branch circuit from the new panel to your other house.