The 'nuke won't detonate' comment is taken here:
http://rezero.wikia.com/wiki/Reinhard_van_Astrea
Q: What if you nuke him? Can that really miss him?
A: The author has stated that the nuke wouldn't detonate.
We just don't have translations for all of the author's interview, so there's that.
As for Pandora: since she has actually never met Reinhard in person, I was power-scaling Pandora's capabilities to Maximized Emilia when she and Emilia fought 1v1 (which was being observed and analyzed by Echidna, the significance of whose views I will explain below). Time and Emilia's recovering power allows her to counteract Pandora's reality-warping influence on herself, and, as her former self, probably even beat her under ideal conditions without being one-shotted by the reality-warp (even though Emilia can't one-shot Pandora either because she's so defensively specialized). I suppose that Reinhard could do better than Emilia at this, we just do not by how much. Still, he would have some level of resistance developed against Pandora - to an unspecified degree, of course, but we at least know now that it can be counteracted by existing means, because it has already been done and explained by Echidna. (raw 'Power' as the most basic form of counter)
[Echidna: Memory alteration isn't your fault. It is Pandora, Witch of Vainglory who can warp reality into her liking. It is both time and your power that weaken her influence]
[Echidna: As you saw, your power is incredible even in uncontrolled state. You could have beat Pandora even when you're young in sheer power. Of course, sheer power alone cannot win the fight, and Pandora is specialized at saving her own skin.]
[Emilia: You suddenly took back your insults.]
[Echidna: I am sympathetic enough to care about someone who just remembered the loss of her mother. Even it's a ***** like you]
[Emilia: Thanks]
http://pastebin.com/XhMY5wg1
So I don't know if Reinhard can always defeat Pandora 1v1. But I find it very likely that he will be resistant to Pandora's power, to a certain degree, that would give him a fighting chance from a theoretical perspective. It's a theoretical exercise because the two hasn't met yet and we can only power scale them to known characters who have already fought.
By the way, if Arc 4 ever gets Manga adaptation, we can also scale Emilia's AP using this in the future.
Which, of course, completely snaps Emilia. She devastates everything nearby with her ice magic unseen of any scale.
I think Echidna's words when she explains stuff to Emilia and Subaru are reliable enough. As an In-Character person she's already the most knowledgeable and omnisicent being in Re:Zero as the Witch of Greed, and she can peer into multiple parallel versions of the past if she wants to.
In fact, I think it's Tappei's intent to use Echidna as a narrator character/plot anchor to explain some important stuffs going on in Re:Zero. She's a good plot device for that task, without having to personally explain those things in interviews constantly.