Sorry to hijack this thread a bit from the CX-9 crowd but I read a few posts in this thread that the 2.5T (GTR and Signature) CX-5 fill and level bolt placement is a huge PITA. I had a spare minute today and I wanted to take a second look for a small bolt I lost yesterday so I decided to investigate (RIP bolty mcbolt, you're gone but not forgotten you POS).
The fill/level inlet imho is not in an awful spot. You still need to remove the airbox and they recommend removing another metal bracket, but you don't really need to remove the metal bracket. You can access it just fine from the front or partially from the side. I removed the level bolt from the front pretty easily and it would be very easy to get a flexible funnel in this inlet.
As for measuring the temperature. I found my bluetooth OBD scanner got the job done with OBD Fusion and the upgraded sensor reading package from OBD Fusion. All in it was like $20 for the bluetooth OBD scanner, $5 for the app and $15 for the upgrade package to support more sensors. It was able to read the ECT temperature just fine. I've attached a few pictures.
Picture 1 is looking at from the air box, the view is partially obstructed by the black tub next to that main electrical connector. You can remove that bracket to gain better access. There's two bolts, the one on the top is easy, the side one is a PITA, would not recommend.
Picture 2 is looking at it from the front of the car. The two black hoses on the left are very flexible and rubber.
Picture 3 is the gauge in OBD Fusion. The car had been on a while as I figured out all the app settings I needed.