If we have P_i=HMAC(I_L,P)*G+P they can't do that.
Did you mean P_i=HMAC(HMAC(secret),P)*G+P and reveal HMAC(secret) for the linkage proof, as you suggested in post #228 ?
So the company wouldn't be able to workaround the issue with Q:=P+r*G because they couldn't compute an x such that P_i==HMAC(x,Q)*G+Q ?