The Everstone was just a given item, it actually has no matter to say in the HG/SS event with the Spikey-Eared Pichu. Regardless of you evolving it or not, the in-game event will still work as the Pikachu/Raichu will still have the given coding to unlock the event if it were still kept as the PC Pichu. :) So, you don't have to worry about evolving it or not, it'll be your choice at the end.Yeah, that's why it's holding an everstone when you get it.
I'm keeping it as a Pichu though as I'm leaving my PC Pichu untouched. I've got three, so I might train one of them, but the other might be kept for trade or just with my second Pichu in case anything were to come up.
Sad to hear it's over so quickly, but upcoming is the Jirachi event coming this February 27th till March 13th. It'll be held in the same place as well, GameStop for the US.