I'd assume that you want to back off about as much as the Peak to Average of the signal, so you need to know that first.

Yes, the general recommendation is to back off the output power by approximately 6–10 dB to avoid intermodulation distortion when amplifying a multi-carrier signal. The exact value depends on the amplifier's linearity and the number of carriers.

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Depends on if you are worried about creating co-channel interference or just distorting your own signal. For the latter, if you can measure EVM on an analyzer then you can generally treat that as an Eb/N0. Say you required Eb/N0=10 dB, then required EVM must be better than 10^(-10/20) = 32 %rms, without taking into account thermal noise.

From a more analog perspective, you can treat the IMD as the noise in SNR and assume the 3:1 drop-off, so a 3 dB back-off would be a 9 dB improvement in SNR; again, easiest to measure on a spectrum analyzer.

For adjacent channel, depends on how close the other receiver is, so bigger problem.