Yes, progressive scan is 'in the player/tv' etc. Have a read of
this. It's very informative.
There may not be any probelm. It may well be that when fast-forwarding, the player doesn't deinterlace (assuming progressive scan is selected), but then, you don't normally watch films at such speed, so there's no problem there anyway.

How did you determine that one DVD was 'definitely interlaced'?
Regards