Took me a bit to even find that effect. Would she permit having you repeat the music track to give you more time. Then split up the sets for each repeat including only enough photos that don't appear too crowded. Also consider making a collage of photos as a still and then transition between several collages. You also can use other software to turn your stills into video, but most won't offer the control that it sounds like you are looking for. Photodex offers some highend options.
The screen display looked pretty grainy, but output was good. Proshow Producer has a Montage effect that might be reproduceable in PD. It does a Montage by having the photo start small and grow bigger as it moves off to the left of the screen. That way no photo is on top of each other, but gives a filling of motion. They also have a Carousel Montage where they rotate around, but the one in front is large, again no overlap on the front photo. They have 1 for portraits and 1 for landscape, which makes me think that maybe by separating the two types into sets you'll get less overlay regardless how you do it.
Another montage is called Parallel tracks of images panning in opposing directions. Like two trains passing. You get 2x the pics in the same time no overlap.
You could also create multiple sets that run the same length of the song, but then put them in a PiP matrix with each video running simultaneously on the screen, but only 1 sound track playing.
Windows 7 Ultimate 64bit
Velocity Micro
Intel Core 2 CPU 6700 @ 2.66GHz
RAM 6GB
NVIDIA GeForce 8800 GTS v258.96
ViewSonic VX2255wm-4 dual extended display
K-Lite 32bit & 64bit
1680x1050
Windows 7 Basic background no Aero
Hitachi 400gb system dr