I bet the Resonance cascade and following Portal storms came in handy, but it is never stated explicitly anywhere that they used them, only that they got alerted by the Cascade or the Defeat of Nihilanth, so I’d go with Pyro:
They have interdimensional portals they can use. Mossman tells us in BME that they can use these interdimensional portals (presumably with high amount of energy needed) but haven’t mastered local teleportation (I think local refers here to our universe or dimension or whatever you wanna call it) so they can’t teleport anywhere on earth or teleport in more forces that are within our universe (stationed on mars, the orbit or the heck elsewhere, could also be that there are no more forces at all).
One attempt to call reinforcements was the Superportal in EP1,EP2 (remember, the energy used was from the collapse of the citadel, so it must be reeeeeally resourceful to open such a portal or send a message through it) but the Resistance managed to shut it down, so I think the Reinforcements are either prohibited by the satellite to open another portal or are shut off temporarily until they have the energy to open another interdimensional portal.
If the latter then --> attempt to go to combine homeworld is absolutely necessary to prevent them from ever coming back again, otherwise it would be a endless circle, only waiting for the the next arrival of the combine.
If the former is true, we would only have to find the Borealis and destroy to cut the combine within our universe off. Remaining Forces on earth aren’t such a big problem I suppose…
As for Xenian Wildlife: I think earths ecosystem and the humans have to get along with them. I think it’s not do-able to get rid of them entirely. After all, they are here since the resonance cascade, so they adapted to earth or the earths flora and fauna adapted to them. They are now part of earths wildlife. I think it’s like with non-native species that immigrate or get imported by humans: Either they adapt to the new environment which comes along with changes in the environment ( possibly extinction for other species) or they themselves die out, but it’s not that nature isn’t able to cope with them.