Thank you to all that responded! We did file the extension based on the end of the month of 9/30/2020 and the extension deadline of 12/15/2020. There was some other language that was not as clear that kept throwing a wrench in the mix. Again, A BIG THANKS ?
Short of spinning off company B employees to a new company B plan (before Company B ceases to exist) and then terminating the company B plan I am not aware of a rule that would let you cash out the terminated employees from the current plan.