I talked to Tommy last night, we went over a few details. I had asked him if the Cinderella could be hooked up temporarily in advance of our April visit and he explained why that'd be difficult (i.e. costly).
He doesn't have a final estimate yet but advised that other bathrooms they've done on island homes cost about 80,000 kronor, so I'm assuming something similar for ours. We'll talk again to finalise all the costs and details, probably when we're there in April.
He said there would be no problem in completing all the work by early June, which is the most important thing.