Is there a חיוב כיבוד אב ואם for Grandparents?

In last week's parsha (Vayigash) the pasuk states ויזבח זבחים לאלקי אביו יצחק. Rashi comments (based on the medrash) that we see from here that a person has a greater chiyuv to be mechabed a father then a grandfather. We see from Rashi that there is a chiyuv to be mechabed a grandfather.

This seems to be contradicted by a Gemara in Makos 12a. The gemara there has 2 different beraisos whether a son can become a גואל הדם to kill his father, one says yes and one says no. The gemara resolves the contradiction by saying that 1 refers to a son being the גואל הדם for his brother which he cannot do and the beraisa which says that he can be a גואל הדם is talking about where a father kills his son and the grandson becomes the גואל הדם. Rashi explains that this is permitted because he is not chayav in the kavod of his grandfather.

The Rama in Siman 240 sif 22 brings down 2 opinions. First he quotes the Maharik who says there is no chiyuv whatsoever for grandparents, the Gra there comments that this is badsed on the Gemara in Makos. Interestingly, the Taz asks on the Maharik that he forgot a Rashi in Chumash. Presumably the Maharik would answer that the gemara in Makos contradicts the medrash. The Rama paskens like the second opinion which follows Rashi in Chumash that there is a lesser chiyuv.

How does the second opinion explain the gemara in Makos? One pshat I saw was as follows. In the gemara in Makos the grandson becoming the גואל הדם is a fulfillment of kibud av v'em for his father (the victim) and therefore overrides the chiyuv for his grandfather. In other words, since the grandfather's chiyuv is on a lower level then the father's wherever there is a conflict the chiyuv to be mechabed the father takes precedence. Here, the kibud of the father is to become the גואל הדם on the grandfather.