Ok I played with that calculator a little. What if I replace the tube from the pump to the bulkhead with 1" pipe and took out the tee and add a y pipe for return? Think it might change it up a little?Assuming that you are correct that at full siphon the 1.25" drain can supply at least 1100 gph, then you definitely need a new return pump. I think you have more head loss than you realize. Try the head loss calc at (Head Loss).
I tried it with the following which I gathered from your post (I may be off)
Vertical: 4
Horizontal : 1
Pipe diameter: 0.75
# of 90 degree: 1 (your T)
# of pipe exits: 2
# of pipe entrances: 1
With the MAG18 it calculated a head loss of 9.5' with a flow of 874 gph
With the MAG24 it calculated a head loss of 11.58' with a flow of 1030 gph
While I do not claim to know the accuracy of this calculator it does suggest you probably have a lot more head loss than you expect. You really need to get the return flow up to 1100 if you want to run your skimmer the way you have it setup in your sump.