If you apply some logic to it, apparently the Fredericks family is hugely powerful, enough to leverage a marriage to the royal family. (You go, Marquis.) They had a huge enough reputation to lead the entire noble faction (ie, the majority of those with money), and were basically more powerful than the royal family. The Fredericks family spent the entire engagement, and probably longer, working to strip Asher of all his power and reputation so that he would HAVE to marry Isis and she would rule as Queen, and the Fredericks family would grow even more important.
With Aria's help, Asher dodged the noble faction's noose, and they began to drain the noble faction's power by empowering the common people, giving rising businesses a leg up. Less money for the nobility meant that they didn't support the Fredericks family, because they weren't prospering during the decline. If they wanted business, they had to do what Asher wanted (thereby giving him more power than the Fredericks).
Isis' current gamble is attaching herself to a foreign King from a VERY wealthy nation, and leaving Asher in the dust before he can ruin her reputation by rejecting HER. Add in that one of the tempations for the marriage is to help that foreign King conquer THIS kingdom and expand its territory (which is why Aria and her mom are worried about war). The noble faction still loyal to the Fredericks family will assist the invading army launch a coup in order to regain the wealth they lost because Asher grew savvy enough not to lie down and die.
Even if Asher predicted this outcome, there won't be much of anything he can do about it, because he has very few powerful allies in his country (only a few of the least rich of the former noble faction, and they probably won't fight for him). Plus he can't complain about a woman he was going to reject marrying a foreign royal, as there is no physical evidence of her future treason.
Would YOU prefer to pay your taxes to a party who helps you gain wealth, or one who costs you thousands of dollars by obstructing your businesses?
And no, the country wouldn't be ruined if everything was quick and conducted with a minimum amount of bloodshed. 5000 armed soldiers suddenly appearing in the royal palace while the royal family are all snug up in their beds unsuspecting, with only a few guards who are fat and lazy from decades of peace keeping watch? That is not a solid position to protect your kingdom from.
If the invading army arrives before the national army can rise up to defend the royal family, there's not really much they can do. The entirely royal family would likely be dead at this point (were they regular people).

Wait.
Why Isis married to another king is a thing..? I mean not about spoiler but more like the logic itself. I know she almost engaged with Asher and she is a marquis daughter and so what?? I really dont get it. ( ̄∇ ̄")