First can Zaamassal use the After Activation effects of a paradigm that he has set up during the After Activation phase?
Yes, that works.
Secondly a really weird situation came up. Zaamassal played Paradigm Shift (I can't remember the style) and Seth played Wyrding Dash. Naturally Seth predicted Zaamassal's base. Seth's player argued the following:
1) Seth's priority was higher and so should get to activate his Start of Beat effect first.
2) He chose a new base that resulted in a priority tie - causing a clash.
Priority is only checked once - between the Reveal phase and the Start of beat Phase. That's the only time a clash can happen.
Once priority is compared (and any clashes resolved), the active player (i.e., the player that goes first) is set for the beat, and cannot change because of further changes to priority.
So in your situation, Seth would be named active player. He would go first in the Start of Beat phase, but when he switches his base, he would still be active player, no matter how low his priority is, and there would be no clash. After Seth is done with all his Start of Beat effects, Zaamassal would get to activate his effects, including the paradigm change. Then the beat would continue as normal (with Seth as the active player).