Professional installers often use a towel to help steady the base. It would be impossible to make a base that fit perfectly with every type of carseat out there.
Yep, our installers told us to come with a towel just in case. We used a towel and one face washer with the capsule, they told us where to put them, and the convertible seat has two towels underneath it.

I'm a bit of a carseat nerd too, and have been watching the ratings for years, because some manufacturers change their design a bit from one year to another, so something can rate brilliantly one year, not so much the next. I've had our carseat picked out since 2008 (we weren't even properly ttc) and I love it.

You'd be able to sell your Unity on and upgrade. People do buy these things secondhand. I wouldn't, but people do. I know up here, local refugee support groups are always looking for secondhand carseats, because sometimes it's that or nothing