Power Amplifier Bypass Switch Convention?
If you want to bypass the PA, you'll need to have a separate RF path. If you just put the SPST in parallel with the PA, you'll be making a very crude splitter with one path that has gain. This will pretty much kill any isolation you have and you'll basically make a feedback loop.

If you want to bypass the PA, you'll need to do something like you suggested with the SPDT low power path. You can probably find a PA with built in bypassing, honestly.

Now, if you have the PA in shutdown mode such that there is no gain, then the feedback and oscillation is not a problem. However, you'll still have a rather large impedance mismatch if you don't design your circuit correctly. The chip you've linked has a 1.5 VSWR when in shutdown, which means that you'll actually split a lot of your power with the SPST approach. You'll need to create that bypass path.