1. yes, you may want to change to a different standpipe or block the grooves at the bottom by gluing an acrylic patch on the inside of the overflow.

2. the water will only flow back to the sump as the level in the tank rises from the return pump, so you should not have to worry about the drain outpacing the return pump.

3. if you have the skimmer output in the return area, and the allows any microbubbles to leave in the drain water they will get pumped right into the display, so i would put the skimmer intake in the return area and the skimmer drain into the first so the filter material you put in the small sections will help take out the microbubbles.

4. from what you have described, you could use one rio for the skimmer and one for the return, just be aware that people (including me) have had issues with older rio pumps burning up and contaminating their tanks or shocking them. not sure what kind of fluval filter you are talking about, but if it is a canister filter, you could run it in a loop in the tank or the sump.