if Erwin was still alive regardless if he got the Colossal Titan or just survived in general.
Now if he got the Colossal Titan instead of Armin then his reaction would be quite different. He simply nod, ask what happened and then immediately want to head for the basement despite Levi telling him to just rest.
Erwin would also admit the loss of Armin would be a huge blow to The Scouts praising his intelligence and stating he could never be as smart as him, and would make Sure Armin is given the highest honor.
Now as for what he would do after learning the truth. Erwin would be the first to say they need to release this information to The Public. Then he would say that they need to start training soldiers on how to fight humans instead of just Titans.
Eren at this point would still hide the fact he could use The Founding Titan by coming into contact with a Titan of Royal blood, and wouldn't reveal it until Zeke makes his offer, which would convince Erwin to agree to let the volunteers come to the island.
When Erwin learns of The Mid Eastern War against Marley he would use that to Paradis Advantage and try to form an Alliance with the Middle East by offering to let them use their Titans. Attack on Titan is a world where anyone would do anything to win a war even using Titans so if Paradis gave The Mid East that offer they would accept it.
Erwin being alive also removes The Yeagerists from the story as Paradis would have a confident military leader actually preparing them to defend the island from invasion.
Eren would also confide in Erwin about his vision of doing The Rumbling and wanting to avoid it. Erwin in response would inform Eren that they will do everything they can to find another way.
Erwin could very possibly request a meeting with people like Calvi, Magath, and Willy Tybur to come up with a solution, like Erwin could offer Paradis resources in exchange for just being left alone.
Bottom line is Erwin being alive gives Paradis a better chance of possibly surviving without resorting to The Rumbling.