1. Can we say that using non-specific generalized chips to store keys is safer as it's more decentralized and harder to backdoor than specialized secure elements? I believe Trezor takes this line.
No.
We have Intel processors, who have this Management Engine that is despised by security-conscious people, AMD likely with the same problem. And Besides, the TDP on these units is probably too much to be viable for a hardware wallet. Maybe you can get away with a Celeron but like I said - Intel ME.
Then you have ARM which might be better but there are many vendors who develop the chips and you have to make sure that they are not woefully insecure hardware-wise. They use less power than x86 and support a lot of existing software.
And then you have RISC-V which *might* be good enough for a hardware wallet but only if you can find an up-to-date version.
Otherwise you're staring down the well of microcontrollers, most of which are not documented completely and may have some kind of hidden vulnerability in them. Not necessarily backdoor, but you saw how Trezor devices had that unfixable flaw before. But crucially only state actors care about these microcontrollers, your average script-kiddie hacker will not write exploits that attack microcontrollers because the normal person doesn't use them.