I would not think it is appropriate to mix systems. In addition to voiding both warranties can't see it being nothing more than a bandaid at best. Fix it properly in my opinion.

You've followed the manufacturer's instructions. Keep doing as you're told.

Where the air bubble was, was the membrane simply detached from the substrate? Or was there a void under the membrane, something that needed filling?

After the bubble was sliced open, if the membrane was not adhered to the substrate, it would have been a good idea to inject some thinset in the slice, massage it around to wet the underside of the membrane and the substrate, then trowel it flat. That'll rebond the membrane to the substrate. That is if it was even unbonded in the first place.

Then add your Kerdi patch to cover the slice.

No need for RedGard over the Kerdi.