Nikea was not part of the Emperor’s plans to begin with, and his hand was basically forced by those orchestrating the Heresy as a means to hamstring the Loyalist legions in both the setup for the betrayal and the fighting that was to come. The Heresy would have looked a lot different if the all Loyalists had their Librarians at the forefront, with their powers unsuppressed.

The Eldar’s fall mostly wasn’t because they were all psykers, it was because they were too advanced and had nothing left to do except be hedonistic in the extreme. That, combined with the fact they are all psykers to some degree is what caused Slaanesh to form. If they hadn’t gotten into all the sado-sexual murder indulgence stuff then there would have been no Fall.