Yes, you will likely have to replace your Britax seats since they are on the wider side. The center plastic hinges will also cause a problem, but there are carseats that sit back from the bight (seat crack) far enough that it shouldn't be an issue. The Clek Fllo is one of those.
I'm guessing that whomever you spoke with on the phone at Nissan did consult with the author(s) of the SnugKids guide. It started as a good idea, but was half-assed in execution. It's true that if a seating position doesn't have LATCH, they don't recommend a carseat be installed there. It's frustrating because who knows how many parents have looked at the guides and made decisions based on inaccurate advice.
However, there is significant seat belt overlap in that position and the center seat belt is over the inner lower anchor on the driver's side. There may be an issue if you install a seat with LATCH behind the driver in that it interferes with the center seat belt. And don't forget the LATCH weight limit of your carseats; the heavier the carseat, the lower the LATCH weight limit, both rear- and forward-facing.
You can certainly give it a try, of course. Not everyone is in a position to get a new vehicle when a new child comes along and you have to do what you have to do. Good luck!