Car Seat Tether Router Complications in Toyota Tacoma 2005, Access Cab

SO I have an Access cab:
I'm on my 4th car seat I've owned and out of the 4, only one properly & snuggly fit in the back seat. This was a Cosco carseat. I just bought a Diono though for all of the great reasons they exist to be... I was trying to install it and the tether strap will NOT route through the router to be tether properly. I've attached a picture of the car manual directing me to route the car seat and how it's actually too big. The way I got around this on the Cosco was that it was a simple enough car seat, I could take off the tether via the back of the car seat, un do it, put it through the router, and then reattach to the car seat. However, with my new Diono 3RXT, this is NOT an option as it is a stronger car seat and its basically sewn in...

How can Toyota have routing spots that don't fit car seats actually through them. I also tried taking the tether out of the new tightening fastener but the thickness stops it halfway through (i tried for a good 15 minutes).

An hour later, I ended up putting the car seat in the front seat temporarily (with the airbag off and seat pushed all the way back per toyota manufactures recommendation/rules). See the orange circled picture I attached.

ANY help would be great. Has anyone contacted Toyota before about this? I'm not giving up my truck or that new car seat... I had to switch though because my 2 year old was too tall for the Cosco. Note, we do have two trucks, our other one is a huge tundra and no car seat problems there. I live in a small rural town in Alaska so its not easy to just go trade in a truck, trying to make the car seat work here. Any tips would be great. I'm thinking of contacting Diono today too to see what they say or if they have other options for routing. With just the shoulder strap and no tether (that's the last method to chose) the car seat moves like half a foot back there.

Side question: why can't we use the seat belt and the latchers with this Diono model? I'll be asking Diono that question. It was very strong in the manual to not do both,..

Another side note:
I've had some car seats to NOT flip the seat down to use, the Diono doesn't say that, and the truck manual says to flip it down. I can't keep up with car seats...