Perhaps you might consider doing the de-interlacing/progressive for web output only when you're at the outputting stage?
So just edit as HDV1080i50 [or ProRes] then when you're done, send the Quicktime file to Compressor and turn on frame controls. Set the 'Output fields' to 'Progressive' and choose one of the deinterlace settings. Maybe test out your settings on a small portion with a lot of movement first. Once you're happy with the settings, send the entire movie through Compressor with your nicely-tweaked settings.
Kai
FCP Editor / Producer with Intuitive Films
http://kai-fcp-editor.blogspot.com
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