if, supposing, in event of, dullness
倘 centers on hypothetical or conditional situations: if, supposing, or in the event of something. The meaning "dullness" is a less common, possibly archaic sense that does not clearly connect to the conditional idea.
倘 is a phono-semantic compound: the person radical 亻 suggests a human-related concept, while 尚 likely provides the sound. The exact historical development is uncertain, and the character is rare in modern Japanese.
A person 亻 standing beside 尚, which looks like a small roof over a mouth, as if someone is considering possibilities under shelter: 'if' or 'supposing' something might happen.
For ショウ, imagine a person supposing they should show up: show -> ショウ, and the person 亻 is thinking 'if I go, I'll show up.'