Correct me if I understand you wrong. You are telling me that I need either turn myself in a killing machine and be able to defeat the Dengi King by myself or the alternative it´s play let´s say some hundred (or thousands) of days doing basically nothing.I can confirm that the Dengi invasion will get some additional attention in 084 - like extra dialogues and more offensive measures from other great houses, as well as optional ways of interfering them.
The MAIN reason Ikaanos fell in your playthrough, however, is that you chose to unleash the horde during a *very* weak period in the newly founded Third republic's history. 4,000 rebels-turned-Skyguard are an easy match for 5,000 dengis. I'd recommend having at least 10,000 Skyguard troops behind the walls unless you plan on killing the king yourself.
Because barring mass recruitment events ( and I have seen none at the moment, and I have seen some that bring down the Sky Guard number by hundreds), with the Republic in Pax Raanica ( before the invasion I was with Control in the high 80s, and going up), the only increase in strength for the Sky Guard it´s the trifle of slaves that I can send to them from the daily bounties, may be one and half or two per day on average.
Not an exciting perspective, really.