Toi-
1. about Lo sisgodidoo–If you are interested in where i am coming from see the radzihner in eyn hatecheiles. Although I may have been wrong about the minhag part, he clearly explains why it is not nogea by techeiles. But either way Lo sisgodidoo cannot patur you from a mitvas asey dioraysa especially if you can get out of the lo sisgodidoo problem by wearing btzineh
2– If nignaz means forgotten then in theory it can be found. Now all Techeiles dudes agree it was forgotten. That is metzious and we do not deny metzious. However, as I have quoted from the maharil previously it is not impossible to find it.
The Maharil seems to think you can find forgotten things. his Lashon in siman 5 of teshuvos is
“…????? ??? ??? ????? ??? ??????? ???? ????? ????, ??”? ???? ???? ??”? ????? ?? ????? ??? ??? ???? ???? ?????? ??? ??? ????? ???? ???’.”
The Chemdas Shlomo ??”? ??’ ? ?? seems to agree with that and says “??? ???? ????? ??? ???? ???? ?? ???”? ?? ????? ?? ?? ??????? ??? ??????? ????”? ?”? ???’ ?????.”
The Malbim in his sefer Artzos chaim (OC 9 41) says the same thing
[????? ???? ???”?].