Isofix seat in passenger seat in 2018 Mini Cooper Clubman

I’m expecting twins & have 2 isofix rear facing car
seats in the back of my new mini Clubman 2018.
No matter what car seat i’ve tried I cannot fit my 5
year olds car seat in the middle. I do not want a
booster seat with no back on it, it wouldn’t fit
anyhow. I’m wondering can he travel in his isofix car
seat isofix anchors already fitted in passenger seat?
Some shops have told me yes but switch off airbag
move passenger seat back as far as it can go, some
have said no wait until your son is 12, manual frm
car seems to say no to rear facing baby seat &
switch off airbag, but yes for toddler until 7yrs old
airbag off until then?! Unsure what to do. Any
advice appreciated. Obviously don’t want to trade
in my 6 month old Clubman!!