(1) Normally the momentary start switch would need the proper load rating to handle the draw of the starter solenoid.
Yes, looking at a high volt/amp switch

(2) The ignition switch is wired to turn off the ACC portion when in START. In the RF harness, that means heater, wipers, radio, gauges are switched off. This to give max battery power while cranking. Would be hard to duplicate that function, although maybe not an issue.
If you really want to be creative find a two poll push button that kills the second leg when the first is on and you might be able duplicate the key switch. Run the second through the acc of the key switch and you still would need the key to run the car.
Also, if you added the push button and wanted to turn off the acc when cranking (if you are testing or its cold or something), turn the key to start and then push the starter button
Yes, my plan would be to have the ignition/key turn still in the circuit. The idea is to turn the key to on then push the button to start.

Here's a except from the RF schematic. Any advice on how to tap into the wiring to the key switch to parallel the push switch (NO) ?

ignition schematic.png