Believability:
1) Is there a chiyuv here to believe the blackmailer’s claim that one box contains non-leshmo matza?
2) How about if the blackmailer claimed one box contained matza that was vadai chometz?
3) What if the blackmailer’s claim is that one box contained traif (or otherwise ossur all year around) matza?
Rov / Chazaka:
1) Do rov and/or chazaka come into play here?
When and if the blackmailer confesses:
1) Do we believe anything he says when and if he confesses as to which box is tainted?
2) Can we trust him that he hasn’t tainted more than one box, or actually traifed up some matzos?
yitayningwut–
The “masim atzmo rasha” I see here wouldn’t be baking shelo lishma, or even making it chometz – it would be sabotaging the goods he’s paid to produce in order to blackmail the owner.
A nafka mina (where I couldn’t claim “ain masim”) would be where he claims he b’shogeg forgot to have leshmo in mind, but now that the deed is done, he wants to blackmail the owner.
As far as palginan, that’s an interesting point. I’m not sure if this would be like the case of eidim who claim they were coerced monetarily (who we don’t believe, thereby showing that someone who claims they omitted an aveira just for monetary benefits isn’t believed), or other cased where we disregard the “passeling” portion of their testimony, and believe a part of it; hence the “palginan”.