SAN,
You're right about the EGPWS.
If you say that there's no radiation on the backside of the antenna and that the nose cone doesn't reflect any radiation back in our direction I suppose I'll just have to take your word - even if it sounds quite unlikely to me.
Right again.
It's not somebody with an alleged radar background on an internet forum, like myself, who can convince you the 'backscatter' from the wx radar into the cockpit is negligeable.
Why not drop in on your radio/radar workshop, which almost certainly will have a field strength meter sitting on a shelf, and ask them to bring it along next time they have to do a check on the wx radar, do some measurements in the cockpit, and explain them to you?
And if they have a suitable wide-band field strength meter, they can even compare it with your cell phone.
Far better that way to explain and comprehend the issues, rather than taking my word for it.