Not "everything". That's a convenient exaggeration that makes the claim seem silly. But of course they demonstrably do engage in this practice.

Yes of course they do and comments like yours are exacltly why there continues to be an incentive to use dogwhistles like this. It's an inexhaustable source of plausible deniablity with a clear channel for singaling.

I think certain influential members of the administration, such as Stephen Miller, are absolutely familiar with them even if Trump himself isn't.

And this isn't even particularly obscure knowledge, it's something that a large number of mass murderers in recent years have made an extensive use of in their messaging: https://twitter.com/originalspin/status/1273653992514412544

And this is exactly how this tactic works. The campaign can send dog whistles that neo-Nazis the world over will immediately recognize and appreciate, and useful idiots can come along and argue that all these blatantly obvious things are just coincidences.