RI HAKODESH-
1. The discussion of the color of the strings was never said on the strings in place of techeiles; it was only said on the strings of lavan. See Mishna Brura 9,14, Yam Shel Shlomo Yevamos 3, Artzos Chaim 9, 5, Chazon Ish 3, 25 and others.
2. You claim there is Bal Tosif if I try to do a mitzvah and it turns out to not be one. So you are suggesting not to do it even misafek.
Can you explain to me why this is different than tefillin that there is a safek if they are kasher, or esrog safek murkav, and every other safek mitzvah where the poskim say you are chayav to do it misafek deoraysa lchumra and aren’t choshesh for Bal tosif if it is passul?
the sefer hachinuch doesn’t seem to agree. He writes in mitzvah 454
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The Riaz brought in Shiltei Giborim 4b of dafei hariph in Rosh Hashona says the same thing that there is no bal tosif if you put tefilin in the wrong place.
Do you have any source that there is Bal tosif on a pasul Mitzvah?
It could be you are oiver bal tosef if you make up a new issur of Bal tosef that the torah never assured.