Say I make an HS mule...

Would the EXP still be capped at 121% even if the HSer wasn't leeching?

Yes. But you need another leecher in that case. The cap is not 121%, it's 110%.

You cannot gain more than 110% EXP from a monster in that white text saying "You have gained Experience (3520)" or something along those lines. You need another text to get anymore EXP, one of those lines is "You have received EXP bonus for party (352)."