Your decision to ctb should be independent of the passing of your pets.
Can you not have the pets put to sleep after your attempt? For example, leave them with your vet and advise him/her not to proceed until a certain time. At least then, you would have the benefit of changing your mind and having your pets around assuming you are unsuccessful.
The top 3 methods -- N, SN, and Exit Bag -- are all proven methods when performed correctly. If you have followed the instructions carefully and prudently, failure is highly unlikely. I am presuming you have sufficient gas, a regulator set to the desired flow rate, and a proper enclosure for the gas, and of course, an uninterrupted period of time to carry through your intention.
With that said and done, may I ask what else can go wrong? Under the current legal regime, unfortunately, only you can engage the gas valve.
Best wishes to you, G