Canonically, the borders are not as foolproof as they should be - Yukari's never fixed the hole Yuyuko formed in PCB, and Maribel can get through the border, as others possibly could.** If things exist in Gensokyo due to people not believing in them, things such as the Game Boy and iPod were items that were lost by someone in the real world, and the owner stopped believing they would ever find it again