- What is the protocol for editing other people's questions; would this edit have changed the question too much to be acceptable?
The help center allows us to add edits to a post to clarify the meaning of the post so long as we do not change the meaning of it. Among ways to determine the poster's intentions is reading their comments.
There is an express help center provision in the editing guidelines that allows people to add information from the comments to the question, so that all pertinent information to the question are found in one place.
- To include additional information only found in comments, so all of the information relevant to the post is contained in one place
Also there are specific provisions in the help center that allow us to clarify the meaning of a post for the express purpose of saving it from closure. I find it strange that this is in the section regarding how to answer, but it is still a help center guideline.
Don't forget that you can edit the question you're answering to improve the clarity and focus - this can reduce the chances of the question being closed or deleted.
It should also be noted that before you get 2,000 rep, all of your edits get pushed into a review queue. People with over 2,000 rep review your edits before it is applied, so if they think your edit goes too far, they'll select the relevant rejection reason.
People who are working the review queue see a set of stock rejection reasons. One of those rejection reasons is that even drastic edits should respect the original poster's intention. This implies that drastic edits are permitted within certain limits. What you need to ask yourself is if you are changing the fundamental nature of the question in a manner that excludes the type of answers the questioner might be interested in seeing.
It's hard to know that that unless you are the questioner, which mitigates heavily against editing the question in most cases and more favorably in the case of leaving a comment as you did.
However, given what you are telling us is based upon a comment the questioner made, you have a basis to believe that this is part of what the questioner wants to learn about in the answers. I would opine the edit you are proposing in this case is probably okay, but you would need to add a note like "Adding information from the comments pursuant to help center guidelines", and it should probably be a word for word copy/paste so that the reviewers don't outright reject your edit automatically as undue meddling.
- Am I right to think that with this edit it would be an interesting question deserving of other answers and/or appropriate for ELU?
I don't know because I don't fully understand why we have two websites, but my guess would be no. The questioner expressed they were an E.S.L. learner and E.L.L. was designed primarily with such people in mind. Whoever migrated the question probably thought the O.P. would be better served there. Also, if the question was not considered merituitous on some level, it should not have been migrated because E.L.L. is not supposed to be a trash can. Many reputable members of English Language & Usage are also members of E.L.L. such as StoneyB, Cerberus, Fumblefingers and Mari-Lou A so if they take interest in the question, they'll answer it there. There are also good members at E.L.L. who might be more able to help answer a question in a way that satisfies the needs of learners because that's what they try to specialize them. If your answer remains the only answer to the question, I think it's unlikely anybody else would have taken interest in answering it if it had remained here.
- This isn't the first time I run into a question that as posed I know the answer to and isn't that good, but is very close to a question I don't know the answer to and am curious about; are there specific criteria or protocols in those cases on whether one should edit it, make a new question, or what?
I would recommend asking a follow up question, mentioning the related question and thoroughly explaining what makes your question different from the original, so that your question does not get closed as a duplicate.