You mentioned ADVERSIS REBUS and (ablative plural of ADVERSA RES).
What do you mean by ADVERSA RES? Adversity in one thing only?
Do you then say:==> "Ex adversa res fortitudo"? to mean (Strength FROM adversity, Strength THROUGH adversity-[One singular thing])
first of all “Ex adversa res fortitudo" is absolutely wrong, because the preposition EX takes always the ablative case, i.e. ADVERSIS REBUS in this context, as I’ve often said in my previous answers.
Therefore the correct sentence reads “Ex adversis rebus fortitudo”, while “adversa res” is a nominative case that cannot be used after the preposition EX which needs the ABLATIVE case.
So, when I wrote that ADVERSIS REBUS is the ablative plural of ADVERSA RES (literally, “adverse/ unfavourable thing”), I wanted to explain that ADVERSIS is the ablative plural of the feminine singular adjective ADVERSA (=adverse/unfavourable ) agreeing with REBUS which is the ablative plural of the feminine noun RES (= thing) belonging to the 5th declension.
In short, it seems that you did not yet understand that Latin is an INFLECTED language with five declensions, six cases (nominative, genitive, dative, accusative, vocative, ablative), four conjugations, rigid rules of agreement in Gender, Number, Case, or Person, and many grammar/syntax rules that MUST be compulsorily observed.
To sum up, I have to repeat that "Ex adversa res fortitudo” is absolutely wrong, while the correct form is “Ex adversis rebus fortitudo”.