Paint buckets and there corresponding lids have a nice seal, not sure on air tight but seems so.

What about getting some of those desiccation packs, cctt? I can't answer any of your other questions, I really don't know, but I'm thinking that if you used what manufacturers overseas use for those products that end up on a boat then you'd be golden and wouldn't have to be so worried.

Otherwise, the pet food totes are supposed to be sealed and air-tight.

When I started using Jacks Hydro and CalNit I purchased the 25 lb bags, which was way too much! I use the quart size canning jars to store the salts in for my weekly rez changes. The CalNit is in a 5 gallon container that has a rubber seal inside the lid. Don't know how airtight it is but I've had no issues with excessive moisture and the humidity in my area is always high. The bucket originally contained worm castings, but it seems to be a repurposed 5 gallon paint bucket.

I'm pretty sure the CalNit is the only one you have to worry about when it comes to moisture.

Don't know about the oven