I personally do not think they're a pair, it's only speculation that Shartan & Andraste were lovers and that might only be Tevinter spreading rumours to discredit her. They could simply friends and nothing more. I also don't see Flemeth as being Andraste but I do agree with whoever suggested that it was possible for Andraste to also have a part of Mythal as Flemeth did or does... again not sure what I saw at end of DAI.
I do believe that if the DR was not preformed than yes Urthemiel's spirit(?) is destroyed as it goes to the Warden that strikes the final blow, destroying both. -That makes me think, because the warden isn't a soulless darkspawn the spirit cannot move on to the next when the warden dies? Is that correct... my god I forget too much now... I might need to go play DAO again... (least there's Alistair there
)
When Morrigan confronts her, Flemeth says she can only take over a willing host so it would never of happened unless Morrigan allowed it. Whether or not that's both Mythal and Flemeth, I don't know...