Seriously? "The Darkspawn are such a huge threat, I ran all the way to Denerim to deal with them." doesn't hold water with me in the logic scale.
It's the one place where one can easily talk to all of the lords and ladies of Ferelden, as they're then amassed there. I'd say the Landsmeet was going to be called anyway, given that a short span of time exists between Ostagar and the scene itself. It's the best way to secure all the forces of Ferelden in one fell swoop, rather then having to deal with the petty grudges each and every lord will have.
"Why should I send my forces to defend my neighbor's lands? Or send them south, so my neighbor to the east can strike at me?"
If there's one thing people are, it's stupid. As that scene showed us, sadly.
And over the course of the year instead of dealing diplomatically, or allowing Anora to do what she does best, he insists he's the only one who can deal with the threat, but quite clearly does nothing as the darkspawn continue to sweep across Ferelden laying waste to whole tracts of the country.
Anora did try to get the lords and ladies to stop their fighting through diplomatic means. Many of those same lords and ladies ignored what she said, thinking that those words weren't her own and were her father's.
Anora was in fact doing as much as she was able to (both because Loghain did put her in a spot, but also because of Ferelden's systemic limitations bound by feudalism). She did damage control and allied with the most powerful person in the political spectrum at that time, hoping this would quell any chaos that might rise.
Siding with the banns on her part would've been wrong to do, as it would've fueled tensions and alienated her from her father and her father's military.
Beyond that, I believe that part of the reason Anora is kept at somewhat arm's length from the throne is due to Howe playing the part of Grima Wormtongue, more or less. Howe holds Anora in contempt (in fact, he's clearly a sexist) and wishes her dead and he manipulates Loghain -- Loghain is certainly a victim to Howe, though that alliance was necessary from a political/military standpoint at the time.
Howe is obsessed with gaining power for himself. I have no doubt he would look at Anora and do anything and everything to get Loghain to believe Anora can't rule at this time -- "She mourns her husband's death my lord. She is too stricken by emotions" -- because it would mean he, as Loghain's advisor, can continue to increase his power.
That doesn't excuse Loghain's lack of going to Anora for aid or advice on these matters, but I do think it at least mitigates it.
Also, Loghain was cognizant of the effect Cailan's death would have. Cailan is an heirless monarch with a wife that is, while admired, not secure in her leadership of the country (and many actually dislike her as well and would love to see her removed from power). He knew a power vacuum would open up so he filled it right away, hoping this would go towards preventing a civil war -- more so if he could get the army to fight the Darkspawn rather then each other. That's why he declared himself Regent, I suppose. Anora's still the Queen and she can't be ousted, though it does look suspicious to many.
Because we have to remember that as far as the survivors of Ostagar are concerned (and those reports are the only ones that Teagan and company have to base their decisions on) Ostagar was lost because Loghain cut and ran. (Yes, I know that Ostagar was lost either way and YOU know that Ostagar was lost either way as well, but even Anora's quote sums up the way Ostagar is seen by the actors who DIDN'T get to see the bird eye's view of the hoard in the cut scene.)
There are numerous accounts from people who fought at Ostagar who actually back up Loghain's account of how the battle was a lost cause.