That's very weird, i've travelled extensively and never had any problems finding threaded type gas cartridges.
If you're travelling through France there are hundreds of decathalon stores around the country they all stock threaded gas canisters.
http://www.decathlon.fr/cartouche-power-gas-100-g-id_8185223.html
As i say personally i've always found threaded cartridges pretty easily, but if you are nervous about finding them then you could take an adaptor.
This one allows you to use the gas canisters that are used to fill up lighters, most news agents will sell this type of canister.
Alternatively there are the type that convert a
pierced type canister to a threaded type
In many European countries the vast vast majority of supermarkets sell these types of canisters.
Can't say as i understand your concern about gas canister stability.
Any stove you use needs to be on level ground, your Whisperlite has a smaller footprint than the feet that come with Jetboils.
In all my years using gas canister stoves i can honestly say i've never had one topple over.
So i think you are over thinking it.
Likewise i think you are over thinking the weight thing.
Your Whisperlite weighs in around 410g packed, add to that say 850g for fuel, that's 1260g
We've already concluded your equivalent gas canisters will be 373g
A Kovea camp 5 remote stove weighs 142g as a comparison, my Kovea spider is only 195g still more than 1/2 the weight of your Whisperlite.
Whisperlite with fuel = 1260g
Kovea spider with fuel = 568g
If you really want to get anal about weight take a look at the Fire-Maple canister top stove at 49g.
Fire-Maple stove 49g + plastic canister feet 29g = 78g and this is as stable as any stove i've used.
Alternatively if your just boiling water take a look at the Jetboil stove range.
Had problems with my first generation Sol Ti, the heat exchanger fins melted while i was cooking beans.
Bought a second cup and have been using that JUST to boil water for over a year without any problems.
The burner, plastic spreader feet, cosy and 800ml cup weigh only 344g
As i say i wouldn't recommend it is you plan on cooking, but for just boiling water it's extremely efficient, extremely fast and extremely light.
373g for you gas canister and 344g for the Jetboil and you have your complete cooking system for around 2 weeks for 717g.
I can get lighter, but then i have to start using smaller cups/pots and this starts getting a pain when cooking.