Looking good Marcus! I put the rubber mat in my truck also. I didn't want to worry about trying to take care of carpet. For what it's worth, I started in the middle and worked my way out. I probably trimmed a couple inches off each side in various places. I think you will be able to work out all of the bows if you start in the middle.
It's cool that you were able to save the factory card board shelf. I ripped mine up a bit taking it out. I haven't put it back in, but I haven't been able to bring myself to throw it away yet either.
I received my replacement seatbelt over the weekend. You are right! It's all about keeping her riding in it.