As my car is a 2 door sports car (VW GTI) and my husband is really tall, we needed a smaller seat and the TF, would not fit in the car.
I realize it's too late, but at your DD's size, she would fit into the TF without the headrest. *If* you still needed/wanted a greater recline, leaving the headrest off saves room. Then, as she gets older, you install the seat more upright. And once the TF is more upright, the headrest doesn't take up any additional space front-back due to the clever design.
Were you trying to install the TF at a 45°?
Just so you know, in Canada, the Diplomat has a 30" RF height limit, only an inch more than your Maxi Cosi Mico.
Britax sometime has silly RF height limits in Canada, so hopefully you'll get more height out if than that, but my 3 year old who can still rear-face in a True Fit would have outgrown the Diplomat rear-facing BEFORE she outgrew her infant seat rear-facing (she had a Graco SafeSeat) and would be really close to being too tall for the Diplomat forward-facing.
So, if you end up needing another seat sooner than you planned, just keep in mind the TF and MR can both be installed more upright than 45°.
One other thing, taller guys tend to sit REALLY reclined in vehicle seats. I had to get my DH to adjust his seating position to be much more upright with the seat bottom further back. This gives him the same leg room, just sitting more upright, which changed us from not being able to fit much behind the driver's side, to being able to fit virtually any convertible at a more upright angle. It does take the guy some getting used to as they will grumble about sitting differently, but in a week and half, DH didn't even notice the change anymore.
I am glad she is comfy and likes the Diplomat though! A happy child makes travelling in the vehicle much easier! :thumbsup: