Tim can move 10 feet north and be able to hit the doppelganger using his reach (over the heads of Breva and Vinny).
If you want Tim to get to the mimic, he will take an AoO from the doppelganger. He's going to want to avoid the magma rocks, which are still a hazard and he has to avoid the columns. He could go the other way around and then squeeze in between the southeastern column and the altar. He would be standing partly underneath the portal (which moved back and then up after the fire demon came through) and he'd have to attack the mimic around the northeastern column. Both he and the mimic would have cover from that position.
The mimic can't make an AoO vs. Tim since it is grappling.
I was going to have Tuck keep firing at the mimic but with its offer of surrender, he'll target the doppleganger instead. He is not at the right angle to be able to hit the doppelganger, so I'm going to have him move into the room.