NMW doesn’t apply to furloughed workers, unless they are still undertaking training while furloughed (but they can’t do anything that provides a direct service to your business). So, 80% of their wage may take them below NMW but this is legal.
You’ll need to add separate pay periods together when you apply for the scheme. So, NMW workers furloughed in March will have one pay period on the old rate, and subsequent pay periods from April on the new rate.
Hope that helps!