(only commenting on #2)
The fundamental point for #2 (SecureBoot) is that it is only as secure as Microsoft is. Many Linux users are escaping Microsoft, so the idea of making your security subject to trusting Microsoft is not appealing. The problem may be that Microsoft is compromised (by a hacker or by executive order / legislation within the US government) or that Microsoft is malicious. I’m not saying that they are, only that you have to trust that they are not, because you sure as hell don’t have any way of verifying it.
(I don’t speak for Purism but) Purism goes even further. They don’t want you to have to trust Purism either. That’s why there isn’t a chain of trust rooted at any company. Not Microsoft. Not Purism. No company. The chain of trust starts (and ends) with the customer.
The other consideration in point #2 is that there is an inherent conflict highlighted by the statement “if you want to modify the boot loader of a system you must have access to the private key used to sign the code”.
On the one hand, it is a good thing that some random can’t replace your boot loader with malicious code. On the other hand, it is inalienable that you as the owner of the device have the unfettered right to change (replace) your boot loader - potentially long after the manufacturer has abandoned your device and/or ceased to exist. That’s what open source is all about.
How this conflict is resolved in practice depends on your priorities. You may put greater priority on certainty and rigidity. Or you may put greater priority on control and flexibility.
Many Linux users are tinkerers and in their eyes there are far too many e.g. phones that have completely locked bootloaders. You run the code, the whole code, nothing but the code … as provided by the manufacturer.
(A locked bootloader coupled with an abandoned device can potentially reduce your security. You are faced with the choice of knowingly having unpatched security holes or binning your device.)
At the end of the day, Purism’s approach is not inferior to that implied by point #2. It’s just different.
Depending on your priorities, you may find that Purism’s approach is superior to that implied by point #2.