Lars has acquired the same powers as Lion, yes? We are somewhat sure of this. The hair-portal thing is a given, but Lion can do some other cool portal-related shit like this:
which appears to be a relatively short-range warp system that increases exponentially when layered; this is what got our losers to the gem base on the moon even though Steven can’t breathe in space.
While this can’t be used to get Lars back to Earth, it sure as shit might be able to get him around the surface of Homeworld without too many people noticing him. What I’m getting at is that he might be able to get ahold of a ship.
Except he can’t pilot one, and if anyone sees him getting into one of those suckers, he’s done for. He also can’t exactly scout out a docking station to tell when a relatively small craft is docked and emptied out enough to be hijacked.
So he needs a gem to come with him to pilot that sucker, and to keep an eye on the docks. The other off-colors can only do so much; they’re awfully conspicuous. Some, like Fluorite, are also friggin’ huge and the odds of Lars fitting them all into one portal is… not great.
So he needs a small, innocuous pilot who is completely at home navigating modern Homeworld.
You’re really gonna tell the Diamonds that you allowed a band of traitorous rebels to infiltrate a highly secure facility and escape from right under your careful watch? Doesn’t sound like a wise thing to do, Holly Blue~