Sorry, your first statement "Nobody ever stopped speaking Hebrew." is historically inaccurate. Most Judeans slowly moved away from Hebrew to Aramaic since Assyrian Imperial times. It's not that difficult if you remember that both were Semitic languages (as was Akkadian) and pretty mutually intelligible. Aramaic however had the advantage of being understood commercially elsewhere, so it spread further (just as English does today)
Koine Greek gave the ability to converse with Egyptians and Europeans, so was also important
you cannot say " At no time did any of the disciples speak or understand any language but Hebrew, and they certainly had no ability or capacity to learn Greek, and why would they?" -- you have no proof for that especially that Aramaic was the official language of the Galilean region from the 6th century BC and was the common man's tongue as it was superseeded among the upper classes by Koine Greek following the Macedonian's conquering of the last Farsi Shahenshah
remember even that Matthew 27:46 eli eli lema sabachthani is ARAMAIC, not Hebrew as in Psalm 22:1 eli eli lama azavtani
>> “Most Judeans slowly moved away from Hebrew to Aramaic since Assyrian Imperial times” <<
.
Sorry, no, that is absurd conjecture that is unsupported by the abundant evidence.