You are misinformed, and what’s worse is that you’re showing confidence in your own misinformation by mounting a defense thereof.
Educating someone who believes to already be educated on a topic has proven to be a futile effort, so I’ll be excusing myself from that nightmarish prospect…
So as a parting note I will clarify my meaning to the best of my ability, for you to do with as you please.
“At least one rectangle is a square” is logically equivalent to any existential instantiation thereof, given that it is an existential claim in the first place.
“At least one rectangle is a square” changed to “at least THIS rectangle is a square” does not denote any logical difference.
Both statements are an affirmation that there exists at least one rectangle that is a square… and nothing more
What worse is when you say “a rectangle is a square” it is no more concretised than when you say “at least one rectangle is a square” so your claim that it’s an instantiation is inaccurate.
If you were to say “Rectangle #21 on page 13 of book 2 is a square” THAT would be an instantiation… provided such a book existed.
In either case, you would not get any contradiction to occur from an instantiation. You would have to show the SAME rectangle that is a square is also not a square for there to be a contradiction.
But you have dodged this point before…
“A rectangle is a square” does not contradict “a rectangle is not a square” unless both statements were referring to the SAME rectangle… or unless both were categorical statements.
Neither of which you have demonstrated to any degree.