Put the water in the return chamber. A gallon or two of water coming in through the return to your display isn’t going to hurt or stress anything out. Wild shallow reefs go through pretty big salinity changes during storms/rain (which in the tropics is pretty much daily), and in protected in-shore reefs they have to contend with run-off from the shore, too. A couple gallons daily or every other day shouldn’t hurt anything (I agree that putting it all in the sump where water flow is low and the volume of water is much less would be more stressful than adding directly to the return/display). You can make a gravity fed DiY ATO for around $10. Just get a float valve and make sure the reservoir is either above the sump or if it’s parallel, that the reservoir is deep enough that the water pressure will force water into the sump (use a five gallon bucket as the reservoir and put the outgoing water hose on the side at the bottom). You’ll need the float valve, some tubing that fits the float valve, a bulkhead that fits the tubing, a 5g bucket, and a drill, $10-$20 tops, and probably safer and more reliable than an electronic ATO. Use two float valves if you want some extra security.
