As the title asks: is it required or recommended before Al Hamichya (or for that matter, borei nefashos as well).
And if yes or no, why or why not?
The Derisha (OC 181 sk 2) writes that the obligation to wash with water only applies to Biblically ordained blessings such as Brikat HaMazon and does not apply to rabbinic blessings. (He doesn't mention if this rule would apply to Birkot HaTorah according to the opinions that they too are Biblically ordained.)
Even so, the Mishnah Berurah (OC 181 sk 23) does note that for all blessings one must make sure his hands are not dirty and he would have to clean them first if they were; however, hands that have no obvious filth on them ("stam yadayim") do not need to be specially washed for ordinary blessings.
So, for Bracha Mein Shalosh or Borei Nefashot one would have to clean his hands if they were dirty from the food (per the Mishnah Berurah), but not necessarily through a ritual ablution (per the Derisha).
No. "pok chazi mai ama davar" (go and see how everyone acts).
As for the reason, if mayim achronim was because of melech sdomis (strong salts), I assume that would only apply to a meal, where they could have used such salts. If mayim achronim is for the bracha, I assume it only applies to a bracha as important as Birchas haMazon, but not a regular bracha achrona. Similarly, there is a halacha to wash before shemoneh esrei, but not before lesser brachos. (While its true we now have a custom to wash in the morning before any bracha, that wasn't the original halacha, and having some food on hands may be a more meikel issue.)