iDVD is only able to produce 4:3 DVDs, just DVDpro is able to set socalled "flags" that make the dvd standalone switch to the widescreen ratio.
it would be possible, to change your clips by adding black borders, so they have the correct aspect ratio for using in iDVD... lots of work.
qt is not a format, just a socalled container; which means, a qt/.mov can contain dozends of diffenret file formats, .avi, mpg2, mpg1, .dv et cetera.
maybe this results to difficulties producing a DVD with iDVD, e.g. an mpg file is playbackable with QT, but in the procvess of converting the the video into a dvd file format, the audio will get lost.
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for problem 1) maybe someon else in this forum can recommend a prg (sizzle?)...
for problem 2) just try, maybe you're lucky and most of your trailers are ok...