That’s not true. US citizens maintain their constitutional rights at the border. That has been tested in court. This is not the case for non-US person.

Unfortunately, this seems to be less true now, depending on which circuit your case is tried in.

Also note that Miranda rights are not constitutional rights.

This guy requested not to proceed without his lawyer present several times, with no result.

> I'm sorry, but the Fourth Amendment does not protect against the intentional destruction of evidence.

It seems to be more true than it did a couple of decades ago, what with more recent rulings like US v. Cotterman and US. v. Koluz better clarifying what counts as a routine border searches vs. more in-depth "forensic" searches that require individual suspicion.

who investigates and prosecutes SS agents who violate constitutional rights?

you'll find it's the same people