You can use the statement showing you owned the shares to claim the shareholder OBC on any booked sailing (that should work for several months in the future as well).
Each ship may have some unique internet access loopholes which may allow some apps to function without a Voom package - you won't know till you try it, but it's not supposed to work.
It is much safer to keep the phone in plane mode instead of risking a sizable bill - not necessarily from the ship cell connection.
Texting via iPhone and iMessage usually works inbound on all ships but outbound (off the ship) usually depends on how the system is configured for that particular ship - it usually doesn’t work on all of them.
We were talking about Voom perks associated with a JS, not whether ships have a Sky JS category. Several ships have the Sky JS category, but only a few of them have the name implying Sky perks. Of the few that do, the perks also include the included Voom package (like the GS and up Sky suites).
I think there were reports of folks being moved from MTD to late dining without notice. I wouldn't pass it by RCI to do would do that. The booking could have been MTD (allowing reservations) and then sometime before the sailing switched to late fixed dining. The purported on board response from MDR staff doesn't make sense though.