If you have filler, say in the cheek area, as it slowly dissolves, will the product slip and pool, creating odd, swollen pockets?

No – if the product has been correctly placed, which is something that any of the trusted practitioners on the website here will be able to do, it will stay in place. If you think your filler is moving from where it should be, go back and see whoever injected it and get their assessment of what’s going on.