IS THE ASSURANCE GIVEN TO THE JEWISH PEOPLE THAT IF THEY BORROW FOR YOM TOV THAT THEY WILL BE REPAID, LIMITED SPECIFICALLY TO YOM TOV AND NOT TO OTHER MITZVOT, AND IF SO WHY?
According to Tosfos (and perhaps all the other Rishonim as well, as I didn't see anyone argue with him explicitly), it also applies to Shabbos. We do not find this promise by other Mitzvos, though.
The reason it applies only to the Seudos of Shabbos and Yom Tov is implicit in the verse which the Gemara quotes, "For the joy of Hash-m is your strength," and as Rashi explains, "The joy that you experience for the sake of Hash-m will assist you to pay back whatever you had to borrow." Measure for measure, the one who borrows will be rewarded that he will not have to experience sorrow (from being in debt) after having borrowed in order to rejoice for Hash-m despite his destitution.
Please note that Tosfos 16a D"H "kol" applies the promise of "Borrow on me" to Talmud Torah and R' Betzalel Ranshburg on 16a cites a Medrash that the same applies to Rosh Chodesh.
Thanks, good point!
The reason Hash-m repays expenses for Talmud Torah is apparently also related to joy -- the joy of the study of Torah -- as Tosfos himself (the one you cited) explains, "Chedvas=Talmud Torah."
Sorrow will not come about due to spending one's last cent to bring one's sons the joy of Torah study. As for Rosh Chodesh, Rav Betzalel himself writes that it is included in Yomim Tovim, since it is also a Mo'ed (which one is apparently supposed to celebrate joyfully).
(a) The Ritvah says all the Mitzvoth-
(b) why than do we say "ase shabattcha chole val tistarechlabreyiot"
(a) True, the Ritva (in the Shitah Mekubetzes) says that money spent on other Mitzvos will be reimbursed. But he does not say that one should borrow in order to perform all Mitzvos, even if one has no money, as the Gemara says with regard to Yom Tov, and Shabbos.
(b) Tosfos 15b (DH Levu) asks your question. He answers that if a person does not find from whom to borrow, he should make his Shabbos like a weekday rather than beg for money to honor the Shabbos.