So “the accused” extends to all non-citizens, particularly those that invaded illegally? If we were so bound, then everyone outside the USA who was attacked by the USA or who attacked the USA in any war that the USA has ever prosecuted is due such niceties. No; with all due respect, I do not believe that’s the part of the Constitution that applies here.
The base case is exactly about whether the defendant is here legally or illegally. Were you accused of being an illegal alien, I’m sure you’d demand your day in court, and have a problem with someone saying “Olog-hai is an illegal alien, let’s throw him out without trial.”
Careful what precedent you set. Democrats might realize they can deport Republicans by just declaring them illegal immigrants.
Hence the importance of a secure border: stop people from getting in far enough and long enough that their legality becomes unclear.