I have traveled extensively worldwide on long-haul flights with an ileostomy with no problems yet. Body scanners? No issues. Once they clock the ostomy bag, they just swab your hands. I have had the occasional pat-down (by female security staff), but nothing that made me feel uncomfortable. I have never been questioned about it (except in the USA by TSA, and that was about 10 years ago and just a query by the guard who felt I was too young to have one!).
On a recent flight last month to the UK via Singapore from Sydney, I was just waved through all security points at each airport with not a worry (even Heathrow was a breeze, which surprised me, being kinda full-on chaos at the best of times).
Never taken a travel certificate and never had to explain the ostomy. I honestly believe no one is interested in anyone but themselves (including security staff!). When I'm traveling, I don't overthink things and just go with the flow. I even enjoy the long-haul flights. What better excuse to binge-watch TV/Movies for several hours!
If you have high output, try and get an aisle seat near the toilets. If I want to slow down output before a flight, I eat a bag of potato chips or pretzels beforehand. Even at 14-hour stretches, I usually only have to use the loo a couple of times. I also eat anything that is placed in front of me on the plane (I'm on holiday and I deserve it!). For me, it is better to eat and drink at regular intervals. Strangely enough, I empty the bag less often when I do so (on or off a plane).
Agree with xnine about doubling up on the supplies (and always put them in your cabin bag/s). I always have double what I think I will need and have never had to use the extras. It's better to be prepared for any eventuality. And they are quite light and don't add much extra weight to your bag. I'm a light traveler anyway and only use carry-on bags.
The sprays you use should be fine in the "approved liquids" ziploc bag you need for other (less than 100 ml) toiletries. That's for international travel though. I don't think it's needed for travel in your own country. I always have stomahesive powder in my supplies, and this hasn't been an issue.
Happy travels for wherever/whenever you decide to head off!
Cheers,
Jo :-)