Berachos 13b Sugya d'Prakdan Lo Yikra Kriyas Shema - Part VII

Continued from last post...
The Mishna Brurah in Siman 239:6 says: "One should make it a habit to lie on his side (even while he's not reciting Shema - just sleeping [Sha'ar HaTziyun] ). It is a great issur to sleep in the prakdan position (siman 63 [Sha'ar HaTziyun]) which is lying either face-up or face-down." (This is inconsistent with the explanation we said in the shittah of the Rambam and the Tur.) 
More bothersome is, that the Sha'ar HaTziyun references siman 63. In siman 63 the Shulcan Aruch says that one may not recite the Shema lying on one's back or face, consistant with his understanding of the Rambam in Hilchos Kriyas Shema. Still, the Shulchan Aruch in Even HaEzer 23:3 follows the Rambam to only forbid sleeping on one's back! Now, however one answers the contradiction, the fact is that regarding sleep the Shulchan Aruch only forbids lying on one's back. If so, how can the Mishna Brurah extrapolate from the Shulchan Aruch about reciting the Shema, that one may not sleep on one's face?
In siman 63, the Biur Halacha talks about the position of the Ramoh in 63:1 who is of the same opinion as the Rabbeinu Yonah. The Rabbeinu Yonah (cited in the Beis Yosef Orach Chaim 63:1) is of the opinion that it is forbidden to recite the Shema even lying completely on one's side, unless it would be difficult to get up - for example, if one is already undressed for the night. It seems that Rabbeinu Yonah learned that when the gemara permitted sleeping while tilted to the side, it is not referring to being a bit tilted, rather, lying completely on one's side. According to this, the Mishna Brurah would make sense, because the Mishna Brurah may be following the position of the Rabbeinu Yonah regarding sleep.
The problem with this is that Rabbeinu Yonah himself (Rif Berachos 7b) says that prakdan means lying on one's back! Additionally, in siman 63, the Biur Halacha discusses the disagreement between the Rambam and the Rabbeinu Yonah and says that one should l'chatchila be machmir like the Rabbeinu Yonah. If so, how can the Mishna Brurah call it, "A great issur," if he's only going according to Rabbeinu Yonah?

A note: B"H I have finished this piece, and will Iy"H continue to the next topic in the next post. I just want to make one comment: It seems to me that I've heard that women are under the impression that they, too,  may not sleep on their face or back. I have yet to see a source that would support this. If anyone can help, please comment!