We have flown with our daughter 10 times between the ages of 3 and 10 months and have never had a problem, and 5 of those flights I was by myself with her. We buy her her own seat and bring her carseat and she sits in it fine for most of the flight (I do sometimes take her out midflight if it's smooth and she's getting annoyed on a long flight), but she's definitely in it for takeoff and landing. She is breastfed, but I have pumped milk to give her a bottle for takeoff and landing, but without it she is fine and has no ear trouble anyway, even when she had an ear infection. (I found out because she would refuse to take the bottle and I was so afraid her ears would hurt, but she was fine, she does have a pacifier, but didn't use it much on the plane.) I'm not one to give medications unless they're absolutely necessary so we never gave Benedryl on a plane-we have used it though when my daughter had a cold and it did not make her sleepy at all, so beware.
I brought a stroller that fully reclined and could face me or outward until she was 9 months, and on that trip we brought our Maclaren Techno Classic, which was much easier to carry around and fully reclines. We brought the infant carseat for the plane, but never used it in WDW as it was just one more thing to carry onto the buses. When you're carrying the diaper bag, stroller, and the baby, there was no way I wanted to add a carseat to that list, and carrying the baby in it was much harder since it was so heavy (we don't use it anymore, but she's less than 19 pounds at 11 months so she was never a heavy baby and it was still hard to carry her around in it). We took a Baby Bjorn, which I love!, and I would carry her in that on the buses. On our last trip when she was 9 months, I was by myself for 2 days and could carry her in the Bjorn, the diaper bag, stroller, and any souvenir bags onto the bus myself.
We stop at a grocery store now that she's on baby food, but before that we just packed the diapers and wipes in our luggage, which left room for souvenirs on the way home. Instead of taking a taxi, get a towncar and they'll stop at a grocery store for you for free, plus they'll carry your bags to the car, which is nice with a baby and a taxi won't do that. We aren't too picky with what kind of baby food she eats (though we buy Earth's Best and Healthy Times at home) so we were satisfied with the selection they had at Publix (Gerber, BeechNut, and Tender Harvest), but we did bring a few favorites from home just in case. We didn't use formula so I can't say if I'd buy it or bring it, but probably buy it down there so I didn't have to worry about carrying it, and if you use liquid you have to worry about it breaking. Also, depending on where you live, all the baby stuff was cheaper for us to buy in Florida than home in DC, so on our next trip in April we might get diapers down there too. We also like stopping at the grocery store to pick up some bottled water. We buy 1 or 2 cases and take a few bottles into the park with us each day, which is a huge savings over buying water at Disney. If you're going to use bottled water for formula, then I'd definitely do a grocery stop. I think all towncar services provide this free, but we've used Tiffany and Cartier towncar services and they both did it. Cartier gives a discount if you book them online too.
We have gotten both pack n plays and cribs, but I think it depends on the resort. At Pop, AKL, and ASMu we had PnPs, but at the GF they offered us either a crib or PnP. My daughter doesn't sleep in either so I never asked about a crib in the other resorts, but they may have them too. They definitely have PnPs though.
Hope you have a fun trip-my daughter had a great time and really started to enjoy the attractions around 6 months old. Be sure and see Spectromagic, that was a huge hit with her.