A lot of work for nothing.
Leave the DV footage as DV, set your sequences to match all the settings. Dragging a clip to an empty sequence will ask you to change the sequence to match the clip this is what you want to do.
Then you need to find the 45 secs of the HD footage and just downconvert it to SD DV to match the other clips and working sequence.
There should be no rendering until you add effects.
You then export this when finished as a self contained QT with current settings.
“The following Media files are not optimized for FCP…” is not an error message, it is just a warning that can be ignored. It happens when the footage used was not brought in by FCPs Log and Capture or Log and Transfer.