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Wonderful! Thank you very much, Martin. Just a picky detail: I think the correct reference for [1972c] should be: D. Pincus, "The strength of the Hahn-Banach theorem", 203-248 in: A. E. Hurd and P. Loeb (eds.), Victoria Symposium on Nonstandard Analysis, Lecture Notes in Math. 369, Springer, 1974 (see link.springer.com/book/10.1007/BFb0065992). I mean, the conference took place in 1972, but the volume was published in 1974 (not in 1973).
Sorry for the delay in replying to the comments. #Soufiane: Let me try to edit the OP and clarify my request. #Gerhard: In fact, I'm mainly interested in the case $n=2$, but I've a feeling that it doesn't make a big difference, and that's why I've phrased the question in its current form. Btw, it is likely that the assumption on $A$ being square and invertible is, on a second thought, completely useless, so I'm going to remove it.