According to the Sefer “V’zos Habracha”:
1. If you ate a regular Mezonos food and rice, the “al hamichya” that you make on the regular Mezonos exempts the rice (פרק כב הלכה ב בשם שע”ת רח/ט, כה”ח רח/מא)
2. The author adds in parenthesis that there is an opinion that one should say borei nefashos first, although this is not the opinion that he is going by and it seems to be a minority opinion. According to that opinion, it would seem that there is some safek involved regarding whether or not the al hamichya exempts the rice. But again, this may be a minority opinion and it may not be one that is accepted l’halacha.
3. Later on (Birur Halacha, 37/2), he writes that if someone ate regular mezonos and rice and he made an al hamichya on the regular mezonos and he wants to continue eating the rice(the OP’s case), it is a safek as to whether or not he can continue eating the rice w/o a bracha, since al hamichya exempts rice b’dieved, so it’s considered that he has a “siluk” (removal) from the rice, and t/f (since it’s a safek), he must do one of the things that one does whenever he has a safek brachos.
Yekke, was this the Sefer that you quoted in the OP?