Yes they do have some propane in them, typically 15% to 30%. You can't get anything more than 30% in the US anymore unless you find some old (discontinued) Coleman Powermax or Bernzomatic PC8 canisters. The presence of propane increases pressure inside the canister particularly in cold weather. The problem with propane is that the higher vapor pressure that it has causes it to burn off faster than butane or isobutane. When all the propane is burned off, you canister may fade out. Running with the canister inverted avoids the issue of preferential burn off, but only run with the canister inverted on a stove designed to do so.I was under the impression that these canisters had a bit of propane in them as well. And that's why inverted canister stoves like the MSR windpro work so well with an inverted can.
You can refill. Not sure if it's worth the hassle or not. But I've refilled with butane which is a perfectly good fuel for use over 50F/10C. I can get 227g of butane for $1.00 whereas a 110g backpacking type canister costs $4.00 to $5.00 depending on the brand. So, refilling gives me a 75 to 80 percent discount. And of course I can top off partial canisters or custom fill a canister for a trip of a given length.I've also heard of people recharging the canisters, but don't really have any experience with that at all.
HJ