[1]
I understand that the Chinuch does not appear to agree with the Shulchan Aruch (Y.D. 61, I believe).
[2]
It may be suggested that Kohanim today are not "Muchzakim" (proven) to be Kohanim with regards to collecting Matnos Kehunah. Since no Kohen can prove that the ZL & K belong to him, the owner of the animal may keep them (ha'Motzi me'Chavero Alav ha'Rayah).
[3]
The CHAZON ISH does add that one should evaluate the worth of the ZL&K before eating it or keeping it for one's self, so that he can give it to a "real" Kohen should he find one.
[4]
I thought the chiyyuv for ZLK falls on the tabbach, not the purchaser.
Nice to hear from you, Sheldon! As for your questions regarding Zero'a Lechayayim v"Keivah (ZLK),
1) The Chinuch has the same opinion as the Ramban, that the current custom not to give the ZLK to the Kohanim in Chutz La'aretz is incorrect.
2) I understand ha'Motzi me'Chavero Alav ha'Rayah when I *may* owe a *specific* individual money. If there is a safek, then the principle applies, and he has to prove that I owe him. But, in the case of ZLK, there is *no* safek that I owe *the Kohanim* the ZLK; it's only a safek if the person who is claiming them is a Kohen meyuchas. How can I apply ha'Motzi me'Chavero Alav ha'Rayah to the *entire* set of Kohanim, when it is *vaddai* that I owe *the* *set* the ZLK? Perhaps that is why the Chazon Ish suggests what you wrote.
3)
Yes, the Chazon Ish indeed asks that it is not proper to turn down the entire Shevet on such grounds (YD 193:12).
4) Perhaps the Chazon Ish is discussing the butcher, not the purchaser, as well.
Be well, and thanks for sharing with us your questions,
-Mordecai