would the negative be, for example, Ils ne seraient pas tellement déçus.

Bonjour Barbara ! The first two sentences are perfectly correct :) However, to ask if someone would be happy to help, although your version is grammatically correct, the exact equivalent wouldn't be colloquial in French. Instead, we'd use "bien vouloir + infinitif", as such: "Est-ce qu'il voudrait bien [m'/t'/l'/nous/vous/les] aider ?" I hope that's helpful! À bientôt !