I'm looking at a Siemens PW1224L1200CU Main Lug Load Center. Will either the 200-amp branch breaker QN2200 or QN2200R fit into this panel? I looked at the wiring diagram but was unsure if a QN2200R breaker is considered a 'QNR' type.
Use the QN2200R (or a Q2200B)
The QN2200R is what is called out by Siemens for horizontal applications in loadcenters and some meter mains, as well as the occasional vertical application with the lugs up. The QN2200, OTOH, is what is typically used in meter-mains that mount the breakers with the handles horizontal, as it provides the correct handle orientation for a vertically mounted breaker in that configuration when the lugs are pointed down (up being ON, down being OFF).
If you don't want to deal with this, there is also a 1x4 double frame breaker in the Siemens catalog, namely the Siemens Q2200B.