CA,
1) Your question was if the person who is saying the “eidus” gets an aveirah. People make mistakes, and considering the type of mistake, they might be a shogeg, oiness or meizid. Saying “eh, I don’t know so why not” is not eidus that the person should be potur on. You were not discussing the person who heard the eidus. And if the person hearing the eidus knew that the mashgiach’s position is “eh” I doubt you can rely on that, because saying, “I don’t know, why not just say ok”, without any basis is not eidus to even say there was an eid echad.
2) If he truly does not know, then it his responsibility to speak to the Rav Hamachshir, that is exactly his job. A Rav Hamachshir (or a reliable one) does train mashgichim to know how to check for tolaim. (How much is necessary and what to look for and the method to look for tolaim) and is responsible to set standards and make decisions when there is a question.
3) I would assume a housewife would know how to check and what to check or ask a shailoh, such as any other shaila as how to check for tolaim. Certainly for those who care about kashrus.
4) Your attitude sums up succinctly why certain people may not want to rely on certain hasgachos or eat at certain places. This does not need more expansion.