Agree with most. You are probably right that Whitlock started because Houck wouldn't get his covid shots and Sale was/is on the IL.
Last year I thought Whitlock was terrific in relief, but he never closed (or almost never) even though Barnes bombed after being terrific, Ottavino closed a bunch of games but not all that well, and even Richards closed a couple of times. I do not think Cora is an idiot, so I have to assume he didn't think--and apparently still doesn't think--Whitlock's niche is as a closer. And, me, I trust Cora's judgment.
In addition, I have this idee fixee on what a closer should be/do: fearless, great command, two really good pitches. And I'm not sure that's a good description of Whitlock. It did however, fit Barnes when his knuckle curve and fastball were working, Kojii Uehara with his slow fastball and nasty splitter, Kimbrel with his big knuckle curve and blazing fastball, and probably Papelbon.
On the other hand, I'm pretty sure that Sale still only uses three pitches: fastball, slider, and changeup (when it's working). So great as he has been as a starter, and I think he was stupendous in his first two seasons in Boston, he just might be the best choice as closer.
Anyway, right now it's Houck because he's earned the shot, despite stumbling tonight. I will be happy to congratulate you when and if Whitlock gets the job and proves he can do it well.