Butyrac is safe on alfalfa - both RR alfalfa and non-RR alfalfa.  Label does not specifically have Waterhemp on it, but label does say control of pigweed, up to 3" tall.   Also, I was thinking it was 30 days to harvest.  However, the label I read says "do not feed straw or hay for at least 30 days after harvest". The way I read that.... is different from " do not harvest for 30 days after harvest."  

I would try Butyrac and do it soon after alfalfa regrows - maybe when alfalfa is 3-4" tall.  That way you will be catching the Waterhemp when it is small as well.

Butyrac label also lists a tank-mix of Butyrac and Butril as a possibility for pigweed.  Rate of Butyrac would be less on the Butyrac when tank-mixing with Butril.  I don't use Butril much.  I don't know if this is a better way to go, than just Butyrac (and Roundup).