I love all-inclusive buffets for holidaying with babies/toddlers. Easy to pick out just the bits you know they'll like. Without having to prep it (or wash up) yourself. Also like PP said, the Ella pouches for back up/travelling are handy.
Most places will supply a cot, but good idea to double-check when you book.
9 months, could well be crawling/ bum shuffling (or even walking/cruising) ... or might not be! Some of mine were, some were not even close at that stage. If she is, you'll just have to be vigilant (like you will have to be anyway) but you'll know how to do that when the time comes.
Car seats can be hired with hire car (but are often a total rip-off price).
Re back carrying, I'm a big fan. We bought a Deuter backpack ( www.amazon.com/Deuter-Comfort-Carrier-Infants-Toddlers/dp/B00HOATAAK?psc=1&linkCode=xm2&SubscriptionId=AKIAILSHYYTFIVPWUY6Q&creativeASIN=B00HOATAAK)&tag=mumsnetforum-21&creative=165953&camp=2025 2nd hand and are still using it now, 8 years and 4 children later and it's still tip top, really good quality, comfortable & adjustable and space for all the snacks/changing bag stuff. You can start using from around 6 months old... and go on using it up til about 16kg I think (if you can still lift it that is - 16kg is MASSIVE - I'm not sure my nearly 5 year old weighs that much yet, but thank goodness we don't carry him any more...)
Most airlines will let you take something like 2 large baby items for free - eg Easyjet you can take any 2 items off list of: travel cot, pushchair, carseat, backpack.... check on the website of whatever airline you're travelling with. And if you want extra stuff, just pay for extra hold items...
And this is great advice from Loudloopy (as is everything else she said) : Don’t worry about the plane, it can’t be helped and it’s not forever