Pushing robots and priority
Option 2 is correct. You say it yourself, priority only controls what order the robots move in. There's nothing in the rules about cancelling a robot's move.
From page 6 of the Wizards edition:

Priority
The priority numbers on each Program card indicate a robot’s
priority for that register phase—how quickly it moves. Whenever
robots are likely to bump into each other, use the priority numbers
to determine which robot moves first. A higher number means
faster movement, so a robot with a 200 card would move before
a robot with a 100 card.

(Emphasis mine)