It's too simplistic to assume because a question may have different answers the question is too broad. Detouring into the issue of whether questions are too broad or not is a red herring that does little and probably nothing to address of multiple answers to a single question. Often multiple answers arise simply because the assumptions necessary to answer the question may lead to different answers. This is a common situation with speculative questions and especially when the answers themselves are speculative.
You only have to look at philosophical questions to see how considering one or two factors differently can result in different, if not radically different answers to the same question. This will not be uncommon here on Worldbuilding SE.
I have found myself doing both. Giving multiple answers to the one question in the text of a single answering post and putting different answers in separate posts. Often there has been little rhyme or reason in doing so. Although if the different answers are sufficiently different it does definitely feel uncomfortable putting them in the same text.
Usually what happens when this occurs, is that I had formulated one answer and during writing the post other possibilities arise and they get added in to the post. If they are too contradictory, this feels like I can't make up my mind. Mainly this springs from the fact that worldbuilding will always be a speculative endeavour. Unless we want to restrict worldbuilding to worlds congruent to quotidian reality. That will reject all questions about magic, future technology, even alternative history, and the majority of topics currently considered legitimate on WB SE.
If any guidelines are needed, then this should be mainly up to the answerers. They should determine if they feel their answers can be grouped together or if they are too contradictory then it may make more sense to post them separately.
If necessary, the OP or people in comments can indicate that if separate answers have been posted whether they should be grouped into the one answer/post. The converse is also true. If the one post gives multiple answers, then the OP and comments might advise that they could be broken up into separate answers.
One problem with multiple answers in a single answers is that, for example, I personally might agree with one or two of those answers and reject others. if that were so, I'd have to either point that out in comments. "I voted for answer X, but am unhappy about answers Y and Z." type of thing. However, after this issue has been raised, I could recommend the poster repost their answers separately.
In summary I believe the only really sensible guidelines would be to encourage WBers to encourage posters whether their multiple answers should be posted as a cluster or separate answers. In the event someone posted another answer to a question, then in that case it makes a lot of sense that latter answers should be posted as a new and separate answer.