Hey there!

I have some questions about this unit and his redirect ability

1- Does this ability get cancelled by Mob Up during the combat phase?

2- If theres no opponent creatures, should I assign the redirected damage to one of my creatures?

Example: my oponnent has no creatures and plays Flames of Tzeencht against my Warrior priest, I'm forced to assign 1 damage to one of my other creatures before assigning all the damage to the Priest?

3- If in a combat a Warrior Priest uses the redirect ability to damage creatures involved in the combat, its considered combat damage? I think not.

Thanks in advance!

1- No as his ability does not cancel damages

2- Yes as it is a forced effect and you have to resolve it (and there is always at least one valid target, the warrior priest itself)
So, yes for your example.

3- No, combat damage are the specific damage, dealt during battlefield phase during assignation and application part of the battlefield phase.

I'm not sure I agree with #3. Redirected damage could reasonably still be of the type that generated the damage. It is being forcefully reassigned, but it was damage pooled from units participating in combat.

If they are still combat damage that mean that they are treated with the others combat damages and there is an action phase between this point assignation and application.

Yes... I'm not sure if there was a point there other than pointing out that there would be an action window between when damage is assigned and applied. My understanding is that all pooled damage gets assigned, Warrior Priests Forced effect kicks in reassigns the first point of damage, and then players may take actions then all damage is applied.