I wouldn't say Mythal is dead so certainly, no matter what theory you buy in to. Flemeth, perhaps. Mythal? Seems hard to kill a god, especially one that's already dead. And survived murder in the past. XD
Sorry but....

..Makes it not a theory
Expanding on zambingo's point - Solas would have started this section before he left, and presumably before he met with Flemeth. Thus, if she turned what he was doing back around on him, as some people theorize, the fresco would be innacurate. Solas would have left this behind as he intended to betray Flemeth, thinking he had the upper hand, when she was the one who would up dominating him or whatever.
Basically, it's a biased depiction.
Also I've seen people interpret this section as the Quizzy's dragon and the archdemon. It kinda of works - Morrigan and Flemeth's dragon look a lot different than the archdemon, and that would be represted here. The would-be horns, the belly scales, the back curving upwards like potential wings... It'd also make sense with the sword, since it's killed not only by Morrigan/Flemeth's dragon, but the Inquisitor's party as well. And that was the plan going into the final confrontation, after all. Use their dragon to weaken Cory's, and ultimately defeat it. So it would make sense for Solas to begin depicting that event.
Whereas, the orb breaking was not something Solas foresaw happening, obviously, and it seems like the ultimate fate of the orb was the entire reason he needed to meet with Flemythal and do whatever. She was his backup, a backup he resorting to taking ony after certain events took place, and after one would assume the work pictured above was started.
If any of that makes any sense at all. XD