LMSC I cannot answer all your points cos I do not really know that much about it. Dave has had his unit 13 years so far. The only maintenance (other than it breaking down) is to remove the filter like you would on a heat pump tumble dryer and clean it once or twice a year. My installer said he has never had a Mitsubishi unit break. He said if it works for 24 hours and has not broken down, then it never will.
If you look at the photos I posted the big box goes outside then in each room that you want it in you have one of the units in the second photo. I have 4 of them so for me being in a bungalow, each one trunks up into the loft, then out to the outside unit. Yesterday I had one of the cassettes running then went and stood next to the inverter outside and made a phone call. It runs near to silent. The bathroom solution would be to put a vent in the ceiling (which I could do as it is a suspended ceiling) then pipe it back from the bedroom unit. I have positioned my bedroom unit above the bed pointing directly at the double doors leading to the wet room so hope that it will blow through. If not I will just revert back to the central heating for the one room whilst we seek a second solution.
I have a 5kw unit in the lounge/kitchen, 3.5kw in the bedroom, 2.5kw in the hall which also covers the second bathroom and a 2kw in the second bedroom. Life span is so far into the future I doubt you or I will be around to see it breakdown! Last night I put it on in the main bedroom for half an hour before we went to bed just as an experiment. It was warm enough within 7 or 8 minutes (I know it was not cold!). You get a remote control with each unit altthough any one remote controls each cassette unit. There is also an ap that the installer is going to show me next week that does away with the remote and you use your phone.
The first firm to quote me, the job was simply one cassette and the inverter supplied and fitted for the main bedroom. They wanted £2600. The chap who did the installation for the same job quoted me £1260. Advice would be find an independent and not a large firm!