"I think that the VP prop is key to the performance of the aircraft and a fixed pitch prop would fail to make the performance numbers."
I'm going to look stupid here but why do you say that? A 100Hp is 100HP. Also the O-200 doesn't develop 100HP anyway. Also the airframe is 20kg lighter when installed with a rotax and you would also be able to uplift less fuel for the same range. Or if your instructing time in the air.
I suspect that the reason the engine uses a wobbly prop was that original conversion was basically a 80HP DA20 powertrain inserted in the the Cessna airframe. And I'm pretty sure that when the DA20 came on the market the Rotax was only available in 80HP and they therefore had to use a wobbly prop to make up for the lack of HP.
I also suspect that even with the wobbly prop the performance on 80HP would not have been Stella hence they upgrade to 100HP.
I suspect there are quite a few reason why they chose the 150 airframe;
Alot more 150 where made than 152
Dirt cheap - especially ones with out of hours engines
Lighter - if what you say about performance is true
152 are cheaper to run in the first place - so less price advantage when converted