On most phones, the indication is in the upper left of the screen. Try it at home - go to a known poor cell reception area but with wifi (maybe your basement) and you should see the phone switch to wifi calling.
Don't try it at sea - as others have mentioned, put the phone in plane mode and turn wifi back on.
Welcome to CC.
Yes, the check in date may vary - it's not always at 45 days out. There's nothing to worry about - you don't even need to check in before showing up at the pier.
Most mobile devices and laptops have a way to send displayed images and video to a compatible device (the TV in your cabin) wirelessly. Usually, it's a function of your device and you'd have to look up how to initiate the casting. Do that (look up how to do it on your device) and if you have a compatible device at home (newer TV), you can even practice before your cruise.
Your cabin steward might also be able to help/set it up for you.
You could probably book all of them associated with the first sailing, but it might be safer/easier to split the air for cruise one with it and then the second two flights with the second sailing. Yes, Air2Sea works with both RCI and Celebrity.
You did help those who happened to read CC pages the couple of days this happened to be on the first few pages of the list of threads. I hope my PPS helps you.
That's kinda the point - many of them can't be bothered to search or look past the first couple of pages of the RCI section of CC so they are unlikely to see your post anyway.