if someone never eats bread during the week, only on Shabbos and he is not sure whether he said Retzai or not, does he repeat Birkhas Hamazon?

Question

The Mishnah Berurah in OC 188:6:16 - קפח,ו,טז says that if someone said Birkhas Hamazon on Shabbos and he is not sure whether he said Retzai or not, he must repeat the Birkhas Hamazon because we assume that [since he is usually not used to saying Retzai in Birkhas Hamazon] he must have not said it.

What if someone never eats bread during the week, only on Shabbos, and he is not sure whether he said Retzai or not, does he repeat Birkhas Hamazon?

The reason why one might have to repeat Birkhas Hamazon even in an a case like this, is (A) Maybe the halacha is univeral, (לא פלוג) - since most people do bentch sometimes during the week also, and they do not say retzai, and thus have to repeat when in doubt on shabbos, there is no personal halacha for those few who only eat bread on shabbos. (I'm not well enough versed in the rules of when we do or don't say לא פלוג.

or

Maybe since the "standard" version of Bentching is without retzai, we assume that everyone who bentches without paying attention, presumably said the standard version. And thus even if this specific persons always bentches with retzai because he only bentches on Shabbos, if indeed he did not recall saying it we would still assume that he said "the standard version without retzai. Is the reasoning why one must usually repeat because "HE" is used to bentching without Retzai, or because the standard bentching is w/o retzai, and we assume that he said the standard one.

NOTE: The Mishna B'rurah cited just says "אם ספק לו אם הזכיר או לא תלינן דמסתמא בודאי לא הזכיר: If he is not sure whether if he said [רצה] or not, without knowing otherwise we assume that he definately did not say [רצה]". He does not say why we assume so. The added explanation about being used to bentching without retzai, is my own understanding of his words, (thus the brackets).

Answer 1

As I’ve alluded to previously in this forum, I’m highly allergic to gluten and therefore can only eat oat if I want to make a HaMotzi. As oat bread is absolutely nasty (and expensive), I only have it on Shabbos and Yom Tov, so this question is very relevant to me. I’ve therefore asked this previously to my Rabbis, and the answer that I got is that there is no difference - I would still need to repeat bentching if I I'm not sure if I said רצה.

One thing which occurred to me after I initially posted this is that I developed this allergy; I wasn’t born with it. Perhaps since I’ve always bentched during the week, this was in fact an issue for me, whereas if I had never bentched during the week perhaps the Halacha would be different.

Answer 2

IMHO there is no difference. Mishna Brura 582, 3 says (about the 10 days of teshuva)

דחזקה מה שהוא רגיל הוא מזכיר

"One is בחזקת that he said as he is used to".

And if you say: that is because the standard is the whole year's version? ==> Mishna Brura there also says that (like in Shul.Ar. 114, 9), with 90 times this חזקה would disappear (unless it is impossible to repeat 90 times). So you see that tongue habitude would win even against the standard.