I'm not sure if you are suggesting that an antenna would have to be detrimental to the radar cross-section of a submarine or if you are suggesting that you can intercept radio waves from a high directivity antenna.
For the former, directional antennas are on all forms of modern day stealth vehicles without compromising the radar cross-section of the vehicle (eg. modern jet fighters).
For the latter I have already seen successful systems such as MADL [1] which use highly directional radio waves to communicate between stealth fighter jets (something you would hope would go undetected). So I highly doubt that submarines couldn't have a similar system in place.
> (2) A parabolic antenna, or any other very directional antenna, requires a stable platform. Either it's on a huge inflatable raft, or the sub is rigidly attached. Either way, the operation will be noticed.
As shown in [1], directional radio waves can be sent from one jet fighter to another. Both of these are not stable platforms.
[1] https://en.wikipedia.org/wiki/Multifunction_Advanced_Data_Li...