The 'חוות דעת סי' קפ"ג ביאורים ס"ק ב answers your question on the Rambam.
A very short summary of his answer would be:
According to the Rambam, Arachin 8a is dealing with trying to determine when a vest can be established.
1 - The Rambam's rule of 7/11 always repeating itself, is only if she does not have a vest. Should she have a vest, niddah days ...
Gra in his commentary to Yoreh De’ah 120:2 explains that this is in fact a difference of opinion between Rambam and Shulchan Aruch, which hinges on how to explain Mikva’ot 8:4:
הָאוֹחֵז בְּאָדָם וּבְכֵלִים וּמַטְבִּילָן, טְמֵאִין. וְאִם הֵדִיחַ אֶת יָדוֹ בַּמַּיִם, טְהוֹרִים. רַבִּי שִׁמְעוֹן אוֹמֵר, יְרַפֶּה, כְּדֵי שֶׁיָּבֹאוּ בָהֶם מָיִם.
I emailed Machon Yerushalayim, and so far no response. So until they do, the following is just educated speculation.
I asked two very smart people for their input on this question, and they each told me their own answer.
One explanation is based on the fact that the Bahag (Hilchos Berachos Chapter 3) rules that besides the mitzvah of reciting the Shema, ...