If the first part of your post is correct (the stipend is for a free will donation) then why the hell not just give a free will donation to a priest or parish??????

There is no way to disentangle the "giving of a donation" and the "getting of a mass said for your specified intention" in the stipend system. The honest assessment of the system is an exchange of money for something of value, and that is bloody well wrong headed. An alternative assessment of the system is bribery--the pretense of giving a gift with the full knowledge that something desired, but nominally unconnected, will happen in return; and that is also blood well wrong headed. 

There is no way to construe this practice as anything other that simony, no matter what "alternate facts" canon law or established practice may pretend to be the case.