This thing is beyond broken in my opinion. All people do is set up sun, which isn't hard thanks to Ninetales, then send Blaziken on something to cause a switch for a free Swords Dance. After that it's pretty much a guaranteed sweep that you can't stop unless you carry Slowbro, or protect against a Hi Jump Kick (or send in a ghost of course). But the sheer power that both those moves carry, especially Flare Blitz, is just ridiculous. This should definitely be banned, if it's not, I'll be amazed.
Ban
I'm not entirely sure what to think about Thundurus. Sure he's fast, but he's also really frail. I've personally only encountered a Thundurus issue a few times while laddering, but I can see how he could be broken. After a Nasty Plot, he hits that massive 698 Special Attack, and that's not even factoring in LO bonuses if the person chooses to run such a set. Leftovers also makes him more bulky than he appears, and prevents some 2HKO's that would be easily pull off otherwise. However, I'm still unsure what to think as I have personally never had a major issue with Thundurus.
Maybe
Reuniclus is so dumb. He ruined the potential that stall had, and he can decimate certain pokemon that you would expect to decimate him (see: non-CB Tyranitar). I find it ridiculous that a psychic type would be so...non-pursuit bait and it's so damn bulky. As you said in the OP, only two things are guaranteed to win against him: Scizor and CM Jirachi. Heck, since Psyshock came around, Blissey can't even wall this little blob.
Ban
No. I don't think Exadrill should be banned. He's non-threatening outside of Sandstorm, and it's not that hard to counteract that if you happen to be running a weather team as well (and c'mon, who isn't). Also, Mach Punch and Vacuum Wave and Aqua Jet all wreck him. And they also wreck other threats, so having to run Mach Punch and the like is hardly centralizing (priority is seriously big this generation, so I see no harm).
Allow
I think he should be banned. As you said, nothing can truly wall him, and even stuff like Ferrothorn lose to the now standard HP Fire. I found it amazing that they even brought Latios down from uber in the first place as I don't think anything changed in preventing Latios from dominating other than the addition of Ferrothorn, which he beats anyway. So I saw no point in him partaking in OU in the first place.
Ban
I don't think this mon should be banned as a lead. Sure it can get spikes up guaranteed (almost), and screens are annoying and all, but it's almost suicidal in most cases as a lead and most people carry a spinner to remove those spikes. Once they're gone, they're gone. However his offensive spread with Psycho whatever (D-Meteor of psychic, I don't recall the name), and carrying Superpower to beat Blissey makes him quite fearsome as well. So I think he should be banned as a sweeper as I find him more threatening in such a role.
Pseudo-ban
---Drought and Drizzle---
I agree with your idea on the weather DA, with the whole one swift swimmer, or chlorophyll user for Drizzle or Drought respectively. A full team is swift swimmers in rain certainly is annoying to face, and I understand the original ban. However once swift swimmer isn't that bad, and it's essentially an Exadrill in the rain, so I don't see why there's a difference. This all applies for the sun teams as well.
Pseudo-ban
---BrightPowder, Lax Incense, Sandveil, Snowcloak under the evasion clause---
Ban - that is all