As snailplane said, RADKFILE is not a source of information on the history of kanji components, their hierarchical decompositions, or their original rationale.
The origin of 叚 is unclear. Its old meaning is "false", "provisional", and 又 is a pictograph for the right hand, often grasping something (indicating tool use, actions, etc.):
You can see in the early forms of 叚 that コ probably comes from another hand, facing the other way (the kind that in most kanji resulted in a ヨ shape):
The left side seems like some sort of cover or container with horizontal markings inside. According to Tōdō's interpretation this could be showing the act of "hiding" items, related to them being "false". Sears thinks it's "a hand with a flint knife striping a skin off an animal", but that feels like a stretch to me; though other people (like Harbaugh) also connect the left part to an altered "skin" (皮). Shirakawa thinks it's an unpolished jewel cut off from a mountain, but Shirakawa is insane and basically always wrong. In summary, there seems to be no consensus.
Since all of 暇蝦霞假瑕葭遐碬豭赮騢 are pronounced カ (with 蝦霞碬赮騢 also sharing ゲ and 假碬豭 ケ), the common presence of 叚 in them is certainly for phonetic reasons.
(All images from Sear's Chinese Etymology website.)