Pleas forgive me for for not asking if you had pryor firearms experience. I just reread my post and felt like my father had typed it, when I was about 9yrs old.
In reguards to the CO2 cartridge still in the gun. I left one of my pellet pistols in my room with a charged CO2 cartridge for about a year, patialy used CO2 had not completly bleed off, so I got a his as I removed it. Some models have a rubber or silicone seal near/around the puncture needle. Your CO2 cylinder could have stuck to this, or some forign material could have collected around the cylinder. But, on the bright side your gun sounds like it survived storage well.
A friend of mine found his faters pump-action .22 rimfire in his shed after the gun had been in a house fire and a leak in the roof near the gun, it was a sad sight. It took two weeks for us to remove the rust/scale and reblue the metal then refinish the weathered stock.
If memory serves me your gun can take an adapter for the CO2 rod and cartridge, should you want to use a larger tank.
Anyway, glad to hear everything looks o.k. with the new toy. Best wishes on a very enjoyable shooting trip.