square…where do I begin.
The rambam in the letter you quoted tells the taimanim NOT TO VIOLATE THE OATHS…why do you care that he calls it “al derech mashal” when he clearly, black and white, says not to break them..this cutting out of a sentence of the rambam is the height of dishonesty.
But chazal already said that wherever the apikorsim find a source for their kefirah, the answer is be’tzidah, next to it. And here, next to the words “al derech mashal” is the rambam’s clarion call never to violate them.
The rambaN also cites the oaths in mamar al hageulah, so any shtikel torahs you can read into the fact that he holds that yishuv EY is a chiyuv nowadays is invalid. Zionists like to say, well, if the ramban holds that the mitzvah is obligatory, how can the oaths be halacha, if everyone does the mitzvah, wont that break the oaths? The kasha is good, but the terutz is krum, The avnei nezer answers the kasha and says that individually, the rambaN holds that everyone should try and go, but Hashem will ensure that not everyone will in practice go.
The oaths are in the gemara, how are you suppposed to ignore them, especially when chazal say that the bnei efraim were killed because they violated them – if it were “just aggadata” (whatever that means) why would the bnei efraim be killed in their violation?
As for why the shulchan aruch and mishneh torah dont quote them, there are many answers. One is that the shevuous arent a special din, but rather part of denying bias hamoshiach; they’re a way of doing so, and not indepedently forbidden. The same way poskim dont have to cite every possible way of blaspheming Hashem’s name; the idea is there already.
As for the rambam’s “al derech mashal” – what he means is that we don’t apply things like ain shavuah chal al shavuah, etc…it wasn’t a literal oath, but rather it was placed on us.