" Does anybody know the purpose of the canister shutoff valve? "

The shut off valve is used to test the pressure in the tank. It's normal position is open. It will only be closed when the PTCM wants to test the gas tank and EVAP system for leaks. The PTCM will direct the shut off valve to close and then draw a very low pressure on the system ( like 1/2 lb. PSI ). If your system is leak free the vacuum pressure is noted, if no pressure the PTCM will recognize a leak and set a DTC.
If that valve is clogged or stuck in the closed position you will draw gas into the EVAP canister.
I believe you can still find unused canisters, but the old one will work. Just let it dry out, blow a little low pressure air through it to be sure it is dry and re-use it. I do not think re-using is a good idea, but it should work. However it will have a reduced capacity.
That entire area needs to be cleaned. You need a new shut off valve and hopefully you can find a new canister.

PS There are canisters with the shut off valve attached on ebay $100 ~ $180