can i subtract audio selected under spectral selection?

It's complicated to explain, but I'm afraid that you simply can't get rid of noise like that. If you subtract noise from noise you end up with - noise! It's all to do with correlation, and that's the reason that Audition has different ways of dealing with it, involving processing using Fast Fourier Transforms on small bits at a time. That's what Noise Reduction does - you take a sample of just the noise, and use that as the basis for the processing.

Yes, you can do this with a spectral selection of just what you want to remove. You can make your selection a noise sample for NR using Shift+P, although you are likely to get a warning that this isn't a complete sample. If you are absolutely sure, then just go with what you've selected. Generally though it's best to let the NR process analyse all the noise in the file, and deal with that.

The only sounds you can remove by inversion are ones where the nature of what you are removing is discrete - so if you had a tone, for example, and you could copy it and invert your copy sample-accurately and place it over the original, then it would certainly reduce its level. How well this would work depends entirely on how the tone was recorded though - if it was with a microphone in a room, then you'd still be left with the room's contribution to it - as that would be uncorrelated.