But honestly, who can ensure to me that there is no national security letter (or other mechanism I don't know about) forcing Apple to cooperate, with a gag order forcing them to keep silent?

Who can ensure me that the NSA et al have are not bribing, blackmailing, or using court orders on the three or four vocal security experts I can name (like Bruce Schneier, tptacek, Moxie Marlinspike, ...). Everything they say on this topic might be manipulated, who knows.

There could be backdoors everywhere, in apps, hardware, routers, lamps, whatever. Occam's razor suggests that this is crazy, but then people found spam sending wifi chips in clothes irons, so I guess nothing is too far fetched.

If you suspect "they" might be out to get you, the only thing you can really do is to stay under the radar, and hope they don't notice you and target you individually.