Well, it is possible some people may have still existed without El Nido, but only since while El Nido natives don't leave the islands in general thanks to the Records of Fate, nothing is said about mainlanders not staying. So for example, from the people we see in-game that live on the islands but came from the mainland, most likely those would've still existed.
About El Nido, no, it couldn't have existed before the Time Crash, since it was a post-timetravel Chronopolis who built them in the first place, except for Gaea's Navel (already existed) and Sky Dragon Island (it's Terra Tower placed mostly underwater).
And going with Radical Dreamers, there was indeed a Serge to exist without an El Nido. Granted, not as we know him, he's quite different. But then again, different time line and stuff.
Kid would exist, and most likely certainly did, as both the CT animated ending and RD showed. Considering it was the result of Schala's actions after merging with Lavos in the DBT, it's result is only dependant on CT's events happening.
