Some Problems in a power amplifier circuit with THS3091
The circuit modifications you have made are trivial, and should not result in any radically different behaviour. However the ratio of the gain setting resistors is different, so you should expect a slightly different gain.

1) That is an artifact of the way the spectrum analyser works. In order to ensure it goes at least to DC, it is programmed to go some way 'below'. -100k is the same as 100k for 'real' signals.

2) Supply currents, especially under signal conditions, are only nominal. 

3) Same as for a voltage feedback amplifier, R4/R3

4) Odd and even harmonics tend to behave differently as they are generated by different mechanisms. It's usual to observe this sort of behaviour.