I agree with your thoughts. my problem is that I have three real contenders, they are all $350/week to rent with no lens. that is one heck of a trial budget (1/5-1/8th the cost of any of the cameras)....I could literally fly to BH for less. obviously can't try for a week at BH but ouch, they're pricey to rent.
So, about renting....
I would probably narrow the field a bit more. I would probably choose my best contender and rent that.
Handling at a store is worth a lot. Downloading some sample RAW files can teach you a lot when you put them through your process. For me, looking at digital images helps me eliminate lenses which don't render in a pleasing way. Adding up the total dollar cost of ownership can be useful (camera, lenses desired, at least one extra battery). Adding up the total volume and weight can be useful. Did I mention handling in the store (hard to say that too often). This set of exercises taken seriously will eliminate contenders or, at least, help prioritize.
I had no trouble getting opportunities to handle all these cameras and take home images I had exposed on my own card at the local store and a couple events.
Do you need a week to test? I lined up dozens of subjects over a weekend rental (I set up a backdrop at a historical reenactment event one day and brought in a bunch of models into the studio for test shoots the next day--while I was photographing one, the makeup artist was getting the next one ready). I also went to a nighttime graduation of a firefighting school (water and fire at night!).
It took some doing to set this up, but simply renting for a week without such pre-rental legwork is likely to end up with less testing, imo.
And talk with your local store. My store will take the rental price off of the purchase price.
To my eye, it looks like Pentax has not been serious enough about MF and has failed to deliver on promised developments. Fuji has been very active with firmware updates, an aggressive lens development path including a roadmap, a clear plan for camera development. Pentax might pick up the ball. Or might not. But Fuji keeps moving the ball forward, as they have done with the X series.
From usability as a camera, Fuji has a well developed interface which is an effective transplant of the system they developed through many generations of the Fuji X line (all the major firmware updates are like a new generation of cameras for most players.)
Hasselblad has an unclear future in MF and is doing a lot of stumbling.
Phase has a strong track record and, like Fuji, keeps at it in an active way. Big. Heavy. Expensive. IQ advantage.
To my view, I would get it down to Phase and Fuji. I would check my budget, which would eliminate Phase for some. I would check how much I want a handy, portable camera (Fuji) against my desire for that last 3% of IQ.
For me, Fuji is an easy winner. You have to clarify your priorities against the available options to find your winner, naturally.
In short, if you do your homework, you can probably get by with a single camera with a weekend rental and your dealer is likely to take it off your purchase
