Twixtor’s built in deinterlacer is no better than AE’s. Revision says so. They recommend fieldskit for deinterlacing.
I believe the keyers can work with separated fields because that is how they are programmed. If you try to key individual fields, the keys don’t seem to work as well. The same with deinterlacing plugins. Don’t ask me, ask the programmers.
The thing you should focus on is de-artifacting, the compression of dv color loss that gets reconstructed. It’s built into Primatte and Magic Bullet. It helps key 10x better. Not to mention not using dv cameras in the first place, lol.