No problem using the car seat. My MINI is about 6 months older than my little girl now. I started using a Prenatal Group 0 seat facing backwards and now a Prenatal Group 1 seat facing forward. Both fit with no problem. I fit them both using the seatbelt.
We use the MINI for family trips every weekend. My girl loves the sound of my tuned exhaust

(she jumps up and down after 4000 RPM, I think I got the next baby Senna

)
Even the trunk has space for either the "sporty" 3 wheel Prenatal (Practic Plus) trolley or the regular and lighter chassis. There's even space left for mom's stuff as well and for a fourth passenger in the back!
The MINI is certainly more space efficient than most people think...
Cheers!!