This ^Didnt address the second half sorry.
Your flow rate will depend on your drain configuration, power of the return pump, and head height of your system.
Youre gonna want a pump that will overperform your goal flow rate, and then use a valve or controller to limit the flow. Many people use 2 pumps for redundancy. I recommend a DC powered pump, as they are quieter, more controllable, and usually more reliable. Lots of people here like jebao DC pumps i think because of their price point. Go for a pump that is at least capable of 2000-3000 gph at 0' head height.
Yes an no. If someones using a valve to go full siphon the the opening is much smaller than a 3/4 inch pipe. But ido see your point. dAnother option would be to forget the e drain and use the 1 inch line as a main drainIMHO 3/4" drain = no drain
They clog soooo fast!