I had to fly a couple weeks in to wearing a walking boot. Although mine was a domestic Canadian flight, the security rules are similar.
Standing and walking without the boot was painful, so when CATSA asked if I could walk without it, I said no. They then asked if I am ok taking it off if I am sitting down, and I said yes. So they explained what the process would be. I walked though the metal detector and then they had me sit in a chair on the secure side and take the walking boot off. A CATSA officer did a wand and pat-down search while I was sitting, including VERY CAREFULLY the injured foot [asking appropriate questions in advance of touching any area etc]. Another officer took the walking boot and sent it through the xray machine that carryons go through then it got swabbed then brought back to me all cleared. I asked and they said they do have a proceedure for if someone can't take off the thing at all.
So a bit extra time at security, but they were very nice and considerate and professional.
A note: if your walking boot has air bladders: physics means as your plane gains altitude those bladders will rapidly inflate. If you don't prepare for this [I didn't even think of it the first flight] it will be VERY painful. Imagine siitting cramed in economy when suddenly there is intense pain from your leg, you remember PHYSICS! and then scramble to bend over and release the air valves as quickly as you can. The solution: once you are seated on the plane [or just before takeoff], LET THE AIR OUT of the air bladders. You won't be going anywhere, but keep it stapped on in case there is an emergency. Let the plane reach altitude, then you can inflate the bladders so you are comfortable and the limb supported. When the plane comes back down the bladders will shink some, so once you are on the ground make sure you inflate them to where they need to be before you go walking on it.
I found that even navigating the airport was a challenge and I requested wheelchair assistance.
There is absolutely no way I could manage Disney by walking in a walking boot. It is simply way too much walking for the poor limb and boot. Instead, I highly recommend renting an ECV. If you don't want to use it all the time, park it in an area and do a few of the nearby rides by walking, but then use the ECV from area to area/land to land. I think you will find yourself using the ECV much more than you think you will. Also consider there are very few benches and places to just sit and rest. The ECV gives you your very own whenever you need it.
Finally, BRING EXTRA SOCKS.
Amazon sells them by the pair. I've been in a boot twice now. The first time I only had the one sock it came with. The second time I was wiser and got another two. MUCH easier and more comfy. At WDW especially you wont want to waear the same sock two days in a row and may even need to swap out the sock mid day depending on how hot and sweaty it gets. You will need to plan how you will wash them [by hand or using the laundry facility at your hotel]. If you do it by hand, they will take a while to dry, so even more reason to need more than one.
Get a waterproof cover for it. There are some nice ones out there. I got and recommend a "My Recovers medical walking boot cover ". Note that it is good for rain but NOT for puddles since it is open on the bottom [this is so the anti-slip bottom of the boot is still exposed so you don't slip]. So have a plan for the puddle of WDW in the rain.
SW