It is clear a non-spouse beneficiary does not require 20% mandatory federal withholding even though they may otherwise rollover.

Is a lump-sum payment to the spouse beneficiary subject to 20% mandatory withholding?

Does it matter if the spouse bene is the sole beneficiary or not?

Generally, the portion of the lump sum being paid to the spouse which eligible for rollover is also subject to the mandatory 20% withholding.

The part that matters is who is being paid.