Your tenacity to stick with this is to be admired BP.
I was in a similar position to you and came up with 3 options. Yours is number 4 I didn't think of
1. Take both cars, I tow the van, missus tows the boat.
2. Do two journeys with one car, missus and dog could optionally stay in the van while I collect the boat.
3. Find cheap but secure boat storage near my holiday destination for the summer months or all year round.
None of these are very appealing but are much preferrable to lumping a darned SIB and enduring all the associated setup effort and performance compromises. It was fun at first but now never again.