No, 100% would be equal to paying someone 50% of market rate. If market is $100k and someone was paid $80k, you could say “paid 80% market rate” or “25% less than market rate” (since a 25% pay increase would bring them to market rate)
No, in the same way that a 100% increase is a doubling, a 100% decrease is to zero.
If you said that the market rate was 100% more than the workers were being paid, that would be correct, but that's a different baseline and not what was stated in the title.
You're mixing up your baselines. I don't know how you got there in your second example. 25% less than "some number" is always (some number * (1 - 0.25)).
