This being said I don’t think this is the metric one should be basing on. We know quite well that ascertaining someones religiosity says very little about his competence etc. So staying focused to try an check the dotted line of מוחזק בכשרות is sort of missing the forest for the trees. (Similar to what you quoted from RYB that halachikly your allowed to but it doesn’t really work. ולענ״ד לפעמים יש פקפוק גם מעיקר הדין וכמו שהארכנו כמה פעמים לעיל.)
+1000
Eirlichkeit and frumkeit does not equal competence or knowledge.
I'm sure it's been posted here before, there was a fellow who sold cookies and cakes in the Mir.
Everyonesome people knew who he was, and I presume many people who bought from relied on him specifically bec he was frum and a yorea shomayim.
After a number of YEARS, this yungerman discovered that halachos of challah. Specifically, the halachos of tziruf of multiple doughs and also that bliya racha can be chayav. Which he hadn't known. And therefore hadn't taken challah from any of his products. In EY.
On the other hand R' Dovid Moore's rebbitzen (one of the respected rabbanim in Brisk) wife sold bake goods for many years (she still might), and they were makpid to have a letter from a Rav attesting to their knowledge in the relevant halachos attached to every basket of baked good they sent out...