If you bring Rav Shach’s opinion on the merits of teaching Chumash in Yiddish, please also quote Reb Yakov zt’l, who told many a rosh yeshiva – “if you want to say a shiur, by all means say it in Yiddish, however, if you actually want to teach Torah, you must do so in the language that the talmidim speak, namely in English”. He also said many times that if the melamed’s own language isn’t Yiddish, he won’t be able to properly teach in it, as he will struggle for words and miss the meaning. There is a reason why Rashi constantly brings Old French, since people had to actually understand what they were learning.
It is OK to want to learn Yiddish and to learn IN Yiddish. Still, if one is going to bring myriad proofs for that point, he should have some intellectual integrity and quote those gedolim who felt otherwise.