You are correct that you used to be able to make a specific ADR for it. As for the current process, I also read the same thing that a regular River Belle ADR should be made & then you’d consult with your server day of to find out if the viewing package could be added on.
I don’t know the answer to the “will it sell out?” question but judging by the few things I read of how the weekend went, I don’t know if Disney really knows these answers either. What I would do if I were trying to do the package is go early in the morning to the hostess stand and ask there if they had any indication either way. BUT I would definitely be reassured that the only people they are handing out those to are ones who are actually dining at the restuarant, so you’ll probably be ok. If you’re planning to dine at 6-7pm they would probably only be able to offer viewing passes for the 2nd showing of the night (even with specific ADRs for the package anything after 4-5pm was designated for the 2nd show with some flexibility but often not- maybe they are more flexible on that now, but I wouldn’t plan on it).