This is a good note--and admittedly I'm not an expert of MI ballot conventions. As reference I used this link to understand what straight-ticket voting means:
From this Q&A what I understand is that you can choose to vote for your Party defaults, and then manually override this for individual races, such as President.
If you assume every registered Republican votes a straight-ticket (as Ayyadurai does) and then overrides their choice for individual races, then the problem reduces to what I explore in this article.
I don't think there's a major contingent of Republicans who don't select straight-ticket options and solely select individual races, which is why I framed the problem as a set of choices between all down-ballot Republicans and Republican President.
However if your intuitions are correct that Individual vs. Straight-Ticket voters are two distinct and large parts of the population, your interpretation and a model built around that would be more correct.