Yes, you cannot. This has nothing to do with you personally. It’s about the community and scientific method. Any person can independently verify the free software (or pay someone to verify) and ring the bell. Anyone who found a bug can spread the knowledge about it and anyone who has incentive can fix it and distribute the fixed version. In this way you do not have to blindly trust the companies but the whole community is able to verify all relevant claims. This provides no guarantee of perfection but it shifts the trust from one single entity seeking profit by all means to the community in which the members can have completely different incentives, including fame, curiousity and usability/security of their own devices.
Such approach makes it much harder and more dangerous (to the company) to hide backdoors in the hardware or software. See also my other reply to you on a similar topic. Transparency is the key.
Same as above: This hardware has open schematics, which “everyone” can check. Also, you can buy Librem 5 USA if you trust China less than USA.
You are saying this as if trusting FLOSS and proprietary software are the same things. They are completely different trust models. For-profit companies pursue profits, which do not necessarily come from good security or privacy. For this reason, I generally do not trust closed software whenever I have a choice.
I never said otherwise.
There is some logic in it, and I would agree if all else would be equal. However here you ignore that you compare many non-profit entities providing FLOSS and single for-profit entity working in secrecy. Consider Intel as a good example why it does not necessarily lead to a good security.
This is a strawman. Nobody ever said that FLOSS provides perfect security. We say however that hiding backdoors is much harder in FLOSS than in closed software. We still don’t know what every Intel CPU runs with unlimited privileges.