I'll give you the recriminations about Loghain's betrayal of Ferelden by shielding Howe, but the rest of it? What precisely does Loghain gain from Howe's massacre of Highever? As was covered above, the Couslands wouldn't necessarily have rebelled against Loghain. And so far as I can see there is no gain at all from Howe's takeover of Denerim; Vaughn shows no indication at all that he blames Loghain for his father's death, even after Loghain's right hand puts him in a position that gives him no reason at all to be generous. Also, my point with the previous post is that there's plenty of circumstantial evidence against your argument that Howe isn't stupid.
Loghain gained plenty from Howe's takeover of Denerim. He effectively controls the nation's capital and largest city which grants access to most of the resources and wealth of the country. In addition, this city is further held by a man who controls Amaranthine and Highever. Between Loghain and Howe, they now control the entirety of the northern and eastern coasts of the country. With Eamon out of the picture and Bryce dead, they would be unopposed in terms of sheer territory in comparison to the bannorn who isn't even united in one single goal (some for Loghain, some against Loghain, others against each other, etc)
Now Vaughn is a real ****** (I just raped your cousin and ruined your wedding, but here's some gold to go away after you've murdered through my castle guards to get to me) so I wouldn't trust him as a reliable source. Plus, he was assumably locked up for about a year while all of this took place, so his lack of information is justifiable.
Remember, the only reason that Howe was discovered was because the Warden happened to stumble onto his dungeon. In short, the man was only caught by sheer accident after getting away with abducting, kidnapping and murdering people for a full year. Morality aside, he was at least smart enough to cover his tracks. You've got me on the embezzling silver part, but everything else suggests that the man was capable at what he did.
It is possible that Howe murdering the Couslands was done completely on his own, as I've said before the circumstances are too fortuitous to have been a complete accident. But apart from the timing and Loghain's lack of reprisal against Howe, here's something else that I noticed.
When a Warden Cousland talks to Loghain, he makes a rather apathetic reaction to learning about the massacre as if he really didn't care. You could argue that he was busy with the battle, but wouldn't that bring out more of a reaction. Cailan was outraged and declared that he'd have Howe hanged while Loghain basically shrugs. You're trying to battle the darkspawn and now one of two teryns who commands one of the largest portions of the army, is killed by another arl in a time of war. Said arl is also missing from this crucial battle (blight or not, you don't want darkspawn in your country)
And all he did was shrug? Hence the possibility of involvement (how involved would vary) and considering what he did to Eamon, I wouldn't put it past him. Especially since there's more evidence to suggest that the Couslands would've opposed Loghain (and would've had strong support from a good chunk of the bannorn) or at the very least, would not have supported him.