Highly integrated RF front-end chipset RFX8055 issues
1) No, it says "Low-Pass Harmonic Filter" which means that Harmonics need to be filtered. Harmonics which are created in the PA. For intermodulation there would need to be a mixer in the path and there isn't after the PA. There are only non-linearities causing harmonics of the wanted signal.

2) When high level input signals are present the LNA could distort and/or saturate and this causes intermodulation products. So if the wanted signal is strong enough the LNA is not needed and can be switched off. This shifts the usable dynamic range up so larger (unwanted) signals are less problematic.

3) Maybe that sentence about "LO rejection" concerns on-chip filters preventing LO signals from reaching the LNA and/or LO signals reaching the PA. But there's no mention elsewhere in the datasheet about LO so it is unclear how effective this "LO Rejection" is. In my opinion you should ignore the "LO Rejection" remark as the chip does not have any LO signal connected to it. Also, it does not "know" what frequency you're using as LO in the rest of your system. I think this is a marketing phrase and should be ignored.