You can reverse engineer the EFI modules, build a whitelist based on known safe code, and then detect subversion at Intel, so this is not a good strategy for serious adversaries.
In theory, sure.
Is it sufficiently simple that people will do it in practice? (I don't know, I expect you would know)
Wouldn't the malicious alterations introduced in a scenario like that most likely be exploitable defects that could be explained away as mistakes? If they accumulate too much around certain people that's suspicious of course, but it seems like it would often be difficult to downright prove that someone intentionally broke the security of a rather complex system.
The point is that it doesn't matter what the typical person will do. All that matters is that somebody, somewhere reverse engineers the EFI binaries, and that it's easy enough for normal people to run a program to check their current EFI against a whitelist of known good EFI binaries.
This is a banal point, except: if the threat is that Intel (or some other huge vendor) backdoors their EFI binaries, it will get out that they did so. It's not "the perfect crime"; it's practically the opposite of that: one guaranteed to be detected, and that will exact maximal damage on the perpetrators when it gets out.
Wouldn't the malicious alterations introduced in a scenario like that most likely be exploitable defects that could be explained away as mistakes? If they accumulate too much around certain people that's suspicious of course, but it seems like it would often be difficult to downright prove that someone intentionally broke the security of a rather complex system.