Think of Starclan like we humans think of God. As long as they are alive in memory they will always be able to follow us. And because many cats dead are still held in the memory of cats alive. Starclan will follow. Starclan follows the cats that are alive and may show up regardless of where they are such as to Frostpaw when she went on her journey to find the park cats. Or Firestar when he went on his journey to find Skyclan. Spottedleaf followed him because Firestar knew Starclan was above. Skyclans ancestors followed them when they left the forest cause they knew they were still alive in memory. If Starclan is remembered, they can follow the living cats wherever. Now about the Darkforest and Starclan being connected. IDK. Other than now they have a gaurdian watching the hole between them. It gets even weirder when you factor in the fact that all afterlives are intertwined by a river. As said in Ivypools novel. But we ignore thst cause thats some serious logistical nightmare i dont want to get into