[Epistemic status: This is mostly hobbyist research that I did to evaluate which tests to buy for myself]
The numbers listed by the manufacturers are not very useful, sadly. These are generally provided without a standard protocol or independent evaluation, and can be assumed to be a best case scenario in a sample of symptomatic individuals. On the other hand, as you note, the sensitivity of antigen tests increases when infectiousness is high.
I am absolutely out of depth trying to balance these two factors, but luckily an empirical study from the UK estimates based on contact tracing data that “The most and least sensitive LFDs [a type of rapid antigen tests used in the UK] would detect 90.5% (95%CI 90.1-90.8%) and 83.7% (83.2-84.1%) of cases with PCR-positive contacts respectively.” So, if a person tests negative but is still Covid-19 positive, you can assume the likelihood of infection to be 10-20% of an average Covid-19 contact.
With regards to self vs. professional testing, there does not seem to be a very clear picture yet, but this German study suggests basically equivalent sensitivity.
You should also make sure to buy tests that were independently evaluated, you can find lists of such tests here or here. The listed numbers are hard to compare between different studies and tests, however, but the one you mentioned seems to have good results compared to other tests.
I am honestly not sure how long the test results are valid, but 2 hours seems safe. I cannot comment on the other numbers provided by microCovid.
PCR tests themselves aren’t that sensitive, as mentioned by atlas. I’ve seen estimates (1, 2) of ~20% false negative. Counterbalancing that is that I assume people who have so little virus in their nose + throat to avoid a PCR test are at lower risk of spreading. But I would (very subjectively) record my microcovids as .3 x raw numbers.
Thanks, this is helpful! Feel free to PM me your payment details so I can send you the $100 reward mentioned in the post.