First of all, mashing an avocado on Shabbos according to the Chazon Ish is forbidden because of tochen, grinding, but R' Moshe Feinstein zatzal permits it because the fruit does not break apart into separate pieces.
Secondly, even of the avocado was crushed before Shabbos there is the issue you mentioned of lash with kneading in the mayo. Some Poskim only permit this when one both changed the order of the ingredients from the standard method and also mixes them in an unusual fashion, and even then some only allow baby food when the child has nothing else to eat. However, other Poskim permit this completely because lash is defined as binding pieces together with a medium, and mashed avocado is already a sticky mass that the mayonnaise only adds flavor and if anything thins out the mixture.
Therefore, while there are grounds to be lenient under extenuating circumstances, it is certainly advisable to make the avocado salad before Shabbos.