Bigger diameter pretty much means more flow in almost any circumstance so bigger is usually always better.

1" sounds pretty good for that size pump, you should be able to find a 1" x 1/2" PVC bushing at most hardware stores, pretty common part but any plastic will work fine. Avoid brass as already stated as it's simply a copper and zinc alloy; copper is poisonous to coral.

pvc sch 40 is what you need, corix is the best place for this. however like said before , increasing pipe size is not necessary. increasing pipe size will give you the same volume at a lower psi, make sure you install a check valve. your volume will be determined by your pump.

put some thought in your return layout and sump size to see if you can avoid using a check valve

I would just go with the 1/2" return, your not gaining anything going bigger pump will only pump what it can.

This is a completely false statement. Pumps will often have smaller intakes and outputs to reduce cavitation and other pressure differential effects but this is independent on the requirements for the pipe network. As stated the larger the diameter of the pipe the less friction. Less friction means more flow. Pipe friction is a major contribution to head loss and you can easily double your flow rate by using 1" pipe over 1/2" pipe.

Also +1 on not using a check valve.