I'm not sure how mobile and Ork Kannon interact. Do I get to decide where to move with mobile first and then Ork Kannon can be allowed to trigger, or does initiative have something to do with the priority here. Also, is there anything about response times that I should know about?

Per the Rules Reference Guide.

After the combat phase begins (step 3.1 in the RRG flow chart), each unit with the Mobile keyword may move to an adjacent faceup planet. This occurs before any reactions to the beginning of the phase.

So as soon as you hit step 3.1, Mobile triggers.

Ork Kannon is a Combat Action and thus must be used within one of the Action Windows associated with the Combat Phase. This action window occurs after step 3.1 (when Mobile triggers) would have completed.

Therefore, all Mobile moves must be resolved before that first Action Window of the Combat Phase.

As mentioned, both accur in a different time, so won't cause any problems.