I completely understand your apprehension about the move to Discussions as well as the upgrade to a newer version of MediaWiki. I'm sorry that you haven't had the greatest experience when it comes to your recent interactions with staff, I hope to be able to remedy that a bit. 🙂
While Discussions is great for certain things, it still lacks a good deal of functionality, this much is true. While there is a big push right now for communities to embrace Discussions, it won't be the only communication solution available to you.
DPLforum will still be available for use on the UCP. DPLforum is an excellent extension that offers all the functionality that you currently enjoy with Special:Forum. If you'd like I can help you get DPLforum set up now so your teams have ample time to use and get familiar with them.
In regards to Message Walls, they'll still be available! Message Walls are simply receiving a bit of an upgrade on the backend, you'll still be able to enjoy them in (almost) the same way you do now! You can read up a bit on the changes to Message Walls here.
Personally I'm looking forward to a lot of the upcoming changes. While it's unfortunate that some features have to be left behind, in the long run it'll make it better for the entire FANDOM community. For example, instead of having to backport changes, FANDOM will be able to update their version of MediaWiki as soon as a new one becomes available. This means less time trying to play catch-up and more time spent building new features.
as well as the upgrade to a newer version of MediaWiki.
Oh, don't get me wrong. I'm very excited about this. It's long past time that Wikia/Fandom got back in line with the MediaWiki core; it's absolutely a move in the right direction.
We definitely won't be using Discussions, so we're looking for other solutions. DPLforum is ancient and would also be a move backward, but at least it's a wiki product.
What we're looking for is a method of communication that works inside the confines of the wiki (not outside as a separate website like with Discussions), uses wiki markup, works with Special:SpecialPages (most notably Special:WhatLinksHere), and alerts editors to new replies to the conversation. What do you recommend?
Unfortunately there aren't any alternatives that meet all the requirements you've listed. While DPLforum does use wiki markup and it works with all special pages, there's no way to automatically alert users to new replies unless you're willing to set up a bot.
On the other hand though, you have Discussions that doesn't work with special pages or uses wiki markup, but it does automatically alert users to new activity.
Sounds like bit of a catch-22, unfortunately. If you'd like, we can look into creating a bot that handles notifications for DPLforum activity, just let me know. :)