I think you should contact the seller and arrange to return or exchange your flask. I would also respectfully suggest that you reassemble it carefully, and not tinker with it any more. It is clearly defective, but most manufacturers and sellers are reluctant to issue refunds on things that have been tampered with.
If you can't return it, I believe Dixie still sells just the flask heads, so you could try replacing the head and keep the original one for spare parts, or try to fix it if you are mechanically inclined.
In any event, I would try to exchange it. You wanted a flask, and you should have one. I have several flask reproductions, all made by Pedersoli, and they are all well finished and fully functional, although I'm pretty sure they were all acquired at least ten years ago. It is my understanding that Pedersoli has virtually cornered the American market for the "clamshell" type flasks, although I believe there are others who make the cylindrical flasks. I believe Hawksley flasks are still being sold by blackpowder dealers overseas, but for some reason the American market for these appears to have dried up. The Hawksley flasks are of very nice quality.
As a rule, powder horns should be air tight, and will keep your powder in good condition indefinitely. I don't think flasks can be made really air tight, because of the valving mechanism. The other thing is that metal flasks may "sweat," meaning condensation can form on the flask, inside and out, with exposure to rapid temperature change. Horn doesn't sweat as badly as metal, if at all. Personally, I don't think flasks are as durable as horns, generally speaking.
For all that, flasks are handy. With practice, you can throw very consistent charges, although it's a good idea to pour the powder from the flask into a separate charger and pour from that into your barrel. I have horns and flasks, and use both.
Best regards,
Notchy Bob