i believe it is very possible that Moridin is a double agent. however, let me add another possibility:

Maybe the Dragon And the Nae'blis are eternally linked in the pattern. but who says they're position are always the same? Ishamael once taunts Rand by saying that in past lives he served the shadow.

The point about the Dark one not wanting to recycle souls that fail was a real eye-opener for me. what if Ishamael and Rand are always there, but they're position is optional. 

if this is the case, then Moridin being a secret agent of the Light would answer another question i've been having for a long time: what makes this Age so different?

if Rand and Moridin are on the same side this time, that may be the deciding factor in the "killing the Dark One" effort.

While I agree with most of what was said here, i would like to touch on a couple of points.

Number two of the posted "impossible things" list: i don't believe that being ludicrous merits being called impossible. Rather, I think Rand already did another impossible thing: he conquered the  Taint. When Nynaeve Delved him in "Towers of Midnight", she found a nexus of light protecting him from the Taint. In that same book, he came to terms with who  he was, in cooperating with Lews Therin, without going mad.

Semirage told him that Lews Therin's presence was a sign of madness, yet he is there in a time when Rand is clearly not mad. in fact, it almost seems as though the Taint waas Shielding Rand from Lews Therin.

So, Rand conquered the madness almost by himself. that is impossible, yet he did it. he also came to cooperate with a past life without going/being mad. that is impossible, yet he did it.