How can I fix a resolution discrepancy between nested sequences?
It's a bit hacky, but I found a solution that did the trick. I opened my .prproj file with a text editor and made these find-replace all changes:

&lt;MZ.Sequence.PreviewFrameSizeHeight&gt;480
changed to
&lt;MZ.Sequence.PreviewFrameSizeHeight&gt;720

&lt;MZ.Sequence.PreviewFrameSizeWidth&gt;720
changed to
&lt;MZ.Sequence.PreviewFrameSizeWidth&gt;1280

&lt;FrameRect&gt;0,0,720,480&lt;/FrameRect&gt;
changed to
&lt;FrameRect&gt;0,0,1280,720&lt;/FrameRect&gt;


If anyone else has this problem and is desperate for a fix, this should do it, although I recommend making a backup first.