From what I've read here, its a bad idea to leave the crickets in the viv - they can bite them while they are sleeping, stress them out, etc.
To keep Tiamat focused on eating, perhaps you could try putting him in a large shallow plastic bin (I'm picturing he's out in open space when you take him out to feed him) where he can't run off exploring, and then drop the crickets and such in there for him to eat. Also, feeding same time every day, if possible, might help to get him to 'expect' the event and what he's supposed to do as well.
I got one of those 3 drawer storage things from Walmart, (the wide one) and use one of them for feeding, (the other two for all the miscellaneous dragon stuff I've accumulated now, so it's quite handy. I lined the floor of the bin with reptile carpet so Joga's got good traction going after the crickets and it all seems to work quite nicely.