I already made the changes to my post. I didn't know Escavaleir can learn Megahorn.First off, Shadow Ball will always do more damage to whatever Reuniclus is hitting over Night Shade, there is no good reason to use Night Shade over Shadow Ball on Reuniclus. Secondly, an all out attacker set is easy to wall, as switch ins other the Ferrothorn will all be faster than it, so it should be running Trick Room on this sort of set (which, to be fair, you did put in the about five slashed options, but it really should take precedence.) Partners for it, I agree on the Trick Room users, but Reuniclus will be needing Light Screen just as much if not a CM set, so it's probably not worth mentioning either, as Reuniclus' bulk generally allows it to survive a few hits anyhow.
Moving onto the counters, most of these are terrible ideas, high special defense won't matter if Psyshock is used (which it commonly is), and just because something has a super effective move doesn't mean that it'll be effective at all.
Escavalier does have a good chance of countering Reuniclus. CB has to be wary of switching in as it could be taken out by the OTR set before it even gets a chance to attack, but otherwise will have no trouble with Reuniclus. SDef Escavalier has nothing to fear from Reuniclus and can kill it at its leisure. The main problems with Escavalier was the move you suggested using with it, X-Scissor. Albeit more reliable than Megahorn, it's damage is pretty pitiful in comparison, having trouble 2HKOing Reuniclus even with Choice Band. Megahorn is the superior choice, and X-Scissor should not be used. Another thing to note is that Escavalier is outclassed severely in OU by Scizor, who fulfills the same role and does it better, meaning Scizor would be a better choice to counter Reuniclus. So, even though Escavalier counters Reuniclus, Scizor does it better.
So essentially, for you tl;dr people, Reuniclus is only countered by two things that you listed, one of those being severely outclassed in the tier Reuniclus is in, so not a very good counters list at all really. The high special defense users have trouble with Psyshock, with even Chansey and Blissey getting 2-3HKO'd, and not able to do much in return as Magic Guard protects Reuniclus from debilitating status, and Seismic Toss can usually only 4HKO Reuniclus at best.