1. There’s a check box in the render settings: you can either render at the project rate then convert to the target rate, or render at the target rate. I urge caution with the second option however: some plugins sound different at higher rates, and not necessarily better.
2. Not sure what you mean by ‘choose to resample’: Reaper just does that automatically right? Anyway, if a clip is being converted to the project rate, and then the project rate changes to match it, Reaper will simply stop converting that clip.