Thanks, Tiggy. Right now, we're trying to install it in the main seat on top. I'll be having an elcs and the plan is to sleep downstairs for the first several nights after we get home, with baby in the carry cot (I had complications after my previous emcs and don't want to risk stairs right away). So not planning to be out and about using it as a double for a little while.
Will add that I'm a bit disappointed that the mattress isn't more padded for a baby. I was hoping to be able to slip the cameleon mattress in for more comfortable sleeping, but the fit is so tight, the mattress bends too much if I try, so that is obviously not going to work after all.
We have got it to fit into the main seat, but as I said, it just seems cramped and not really flat, so I wondered if we were doing something wrong.
I know Britax say to install/remove the cot without baby in it -- guess I was just spoiled by how easy the Bugaboo cot was to swap in and out when DD was born.
As for the velcro pad -- it doesn't come out. I guess it's just there for support.
I'm hoping it will work out and not be a huge hassle or annoyance for the first five or six months when the baby needs to be in. Afterwards, hopefully, it will be easier to use with the two seats than with a seat and carry cot.