Stand's looking good so far. :thumbup:
You could beef up the front top cross support (another 2x4 underneath the current one) and make the center upright removable on the front the stand. It also looks like you could also throw in a door, or have a removable panel on one of the sides.
Dart return pump sounds like a good choice. I would think you could power both the return and skimmer, might take a lot of adjusting but with gate valves it might not be too painful.