US President Donald Trump's team has ordered the termination of vital support for F-16 fighter jet jamming equipment that Ukraine has. What is important to understand is that this could deprive the Ukrainian Air Force of its most important air countermeasures.
They absolutely do. Imagine if the US invaded Greenland. The Danish (and European partners) F35s would be switched off for sure.
Note: Iran still uses F-14 Tomcats.
Possibilities for implanting a logic bomb are endless, but there are still no case reports about an US-originating plane becoming remotely disabled.
(Russians would absolutely like to listen to a message which accomplishes that, meanwhile allies would definitely want ot change encryption keys on any plane which is supposed to receive that. I also don’t think that military planes will accept unencrypted messages. And their communications subsystems are separate subsystems, which can be disabled or replaced.)
Meanwhile, in a hypothetical doomsday scenario, Danish F-35s would land (or be unloaded from a container) in China, and be greeted with cheers, red banners and golden confetti. Damage to the US would far exceed anything that Denmark could do by firing something.
Note: these are scenarios which are not supposed to happen, but dramatic loss of trust among allies can actually result in that - if country A backstabs country D, there is nothing that really prevents D from betraying A to C.