Was Miriam a heretic?

The Rambam counts as one of his 13 ikkarim the idea that Moshe Rabeinu was supreme among prophets; there can never be a prohpet equal to Moshe in stature.

At the end of Parshas Beha'alosecha the Torah relates that Miriam criticized Moshe Rabeinu's separation from his wife, equating Moshe with other prophets who did not take upon themselves such "chumros". Hashem punished Miriam with leprosy and told her that Moshe is in fact not like everyone else, hence her criticism was unjust.

The Rambam writes (Tumas Tzara'as 16:8) that we see learn from this episode the severity of lashon hara. Even though Miriam had watched over Moshe as an infant, had only the best intentions when speaking, she was nonetheless severely punished.

R' Elchanan Wasserman asks (Koveitz He'oros, Agados al Derech haPeshat 12:7) how this parsha can be the basis for a moral lesson about everyday lashon hara. Miriam's offense was unique in that aside from being damaging speech, it also undermined the cardinal belief in the supremacy of Moshe as a prophet. R' Elchanan goes a step further and notes that it seems in fact inconceivable that Miriam should have not accepted one of the ikkarim, the rules that form the very fabric of Jewish belief. Was Miriam a heretic (chas v'shalom)!?

R' Elchanan answers that Miriam was of course not a heretic because it is only through G-d's response to her offense that we learn that Moshe cannot be equated with all other prophets. Miriam had no way of knowing this basic tenet of belief before it was revealed in response to her offense.

I don't know if this is a question or an observation; take it however you like it. The Sefer haIkkarim (I:3) writes that the belief that no prophet can be greater than Moshe goes hand in hand with the belief in the eternity of Torah (he does so far as to ask III:20 why the Rambam counts them as 2 principles when they really are one in the same). Belief in the supremacy of Moshe excludes the possibility of another prophet arising and undermining Torah law. It follows that if Miriam was unaware of the supremacy of Moshe as a prophet, she was also unaware of the immutable nature of Torah; her "sevara" allowed for the theoretical possibility of new laws transmitted through other prophets. I don't know why, but I find this hard to digest.

Still more difficult, the Sefer haIkkarim (III:20) writes that the supremacy of Moshe as a prophet is established much earlier in the Torah. Moshe asks G-d while praying on Sinai "V'niflinu ani v'amcha" (Shmos 33:16), that both he and the Jewish people should be granted a special unique relationship with G-d. The pasuk is a double-request: a request that the Jewish nation be forever unique, and a request that he, Moshe, forever remain unique among prophets -- "ani v'amcha", I, Moshe, and the people, should each be unique. In defense of R' Elchanan, perhaps G-d's answer to this request was not publicly known until Miriam's offense, but that begs the question of why Moshe would keep to himself until this episode knowledge of a cardinal belief that establishes the eternity of Torah.