EVAP CELs, puking gas from filler and slow fillups

So my '13 MS3 has had various intermittent EVAP CELs over the past year or so. P1450, one for gross leak and one other, I think. (Sorry, it's been a long time and I didn't write them down). P1450 is the only one that occurs recently. I've replaced the gas cap with a generic aftermarket and an OEM Mazda and that never seemed to do anything. Yes, I click the hell out of the gas cap when I tighten it after filling up. I've also tested the purge valve solenoid under the hood and it seems healthy.

The CEL's aren't really the issue though. They're just a minor nuisance that I clear out with my AP. The real problem is that fuel sprays back out of the filler neck when I'm trying to fill the tank. Here's the pertinent facts:
--The tank is always under 1/4. Never tried at other levels.
--This happens at all stations and regardless of what I do with the nozzle.
--Weather and temperature seem to be irrelevant.
--When I first remove the gas cap, nothing weird happens. I can hear a little bit of air moving.
--The first gallon or so will go normally at the whatever rate the nozzle is set on then the pump clicks off, I hear a gurgling and a pint or so of gas will spray out and run down the fender.
--The second gallon will usually do the same thing. The spraying out usually only happens twice.
--Filling the rest of the tank takes forever because I'll get a few tenths of a gallon in then the pump clicks off.
--If I pull the nozzle out and wait a few seconds, I can get maybe a half gallon in at one go before the pump clicks off.

What I'm really confused about is the source of sufficient pressure to blow so much gas all the way back up the neck and why it only happens a couple times when the tank is near empty. At this point I welcome any ideas or speculation because I don't know WTF is going on or how to troubleshoot this.

 

Usually fill issues are caused by problems with the charcoal canister or its plumbing.
You're providing the pressure by adding fuel to the tank when the charcoal canister isnt venting properly..

 

There have been a few threads with similar problems. I had this in my old car. My solinoid was bad on the canaster. There is a vent solinoid that allows the gas vapors into the charcoal canaster so they can be cleaned before going into the atmosphere. This happens when pressure builds like when it gets hot out or when you add a bunch of gas.

 

So on a Pu, the charcoal canister is up under the rear subframe, right? Can it be accessed from under the rear seats? Is the vent solenoid under there or under the hood?

 

Updating this thread for the sake of anyone searching in the future. I can't directly confirm this without dropping the rear subframe (which I'm not going to do) but the evidence indicates my charcoal canister has an internal blockage which prevents air from flowing from the tank vent input port to the filter outlet port. If I pull the hose off the inlet side of the purge solenoid under the hood while filling the tank, I can feel air flowing from the hose and the tank fills normally, no puking, at a normal rate.

So the short term solution is just to disconnect the purge line under the hood when I fill up. Easy enough. The proper solution is to replace the canister but since I don't hate myself enough to drop the subframe, that's not happening. I think the actual solution is to put a tee and check valve on the purge line (either back by the canister or under the hood) so that the air displaced from the tank can vent to atmosphere while filling but without introducing any vacuum leaks to the intake manifold when the engine is running.