Morrigan herself said that she was wrong after Flemeth took Urthemiel"s soul. I don't know why this is so complicated to understand or maybe I misunderstood that scene?
She said she hadn't understood everything. Nothing in the preceding conversation indicates that Flemeth hasn't raised and treated the other daughters the same way she raised and treated Morrigan, except with the intention of actually obtaining their bodies rather than the old god soul. And it definitely confirms that she bred Morrigan to be a cynical, friendless (b/w)itch for the exact purpose of whoring her out, using her as an incubator and stealing her child all the while bearing Morrigan herself no ill will. Yeah, totally not the plan of an amoral monster.
Which doesn't make much sense because the more time pass the more the blight grow stronger.
I don't think Orlais or the GW had hard times in cross the borders if they wanted to support the nobles who were against Loghain.
The Empress was most likely waiting for the Blight to destabilize Ferelden thoroughly enough for them to retake the territory after the possibility of a peaceful takeover through marriage with Cailan fell through, and the First Warden has everything to gain from making an example of a pretty inconsequential country to show what happens when you don't support your Grey Wardens properly just because it's been a little longer than usual between Blights.
Ferelden is pretty sparsely populated and surrounded by sea on three sides, and the political situation and lack of Wardens made it obvious that the country was going to collapse relatively quickly. I don't blame the other nations for believing that the Blight could be contained there for a while so much as being willing to actually let it happen. And yes, they could definitely have crossed the border if they decided to.
Doing so forcefully would have marked them as intruders though, and made it so much harder to justify "staying over" after the Blight as other countries would see it as Orlais overtly taking advantage of the crisis, a dangerous and provocative precedent to set. Waiting for the situation to become desperate enough for Ferelden to either ask for the help or just be happy to be rescued would have given them a much stronger pretext for taking suspiciously long with the clean-up.
Of course, it's entirely possible that if Loghain hadn't made those efforts to secure the border Orlais really would have invaded with their Wardens.