So I bought the Samson USB diaphragm condenser and compared it to my NTG1 in the worst possible environment that I could think of for testing purposes: my garage.
Even without the vocal filter, the diaphragm mic picked up much less echo. With the filter, I only noticed echo if I really listened. In a normal room, even untreated, I think it will be ok. The USB mic had a terrible noise floor a higher gain, though. I'm ditching it and getting a similar XLR model with a couple BLUE XLR/USB preamps so I can get higher gain without the noise.
I've got a dual boom mic stand. I think I'm going to just use both mics for all ADR sessions. If I can set up a recording environment at the actress's home with passable acoustics, I'll have the NTG1. If not, I'll have a secondary setup that sounded OK even in the worst possible environment.
Thanks for the advice, all.