At a minimum, I think we need to close off the questions that are asking about structural modifications. Personally, I'm neutral towards whether we close them after updating the on-topic help page, as a custom close reason, or as duplicates of a canonical "You need to hire an Engineer" type question. For that matter, I wouldn't object to closing them purely as "too broad" either.
If I had to pick one, I would lean towards updating the on-topic page to indicate that questions requiring a licensed Engineer to answer are simply too broad for the scope of the site and therefore off-topic for the site.
Regarding answering the questions, even if it's to the effect of "You really, really, really need to hire a licensed Engineer." I'd discourage answering those types of questions as the answers don't provide lasting value for the site.
By way of example, let's take the answer from one of the example questions you listed. It's a well written answer, and currently has 8 up votes. But the crux of the answer boils down to this:
NO! GOD NO! WHAT ARE YOU DOING!
I don't think we want answers like that to become commonplace on the site.
A cleaner way of handling that would have been to leave a comment stating "There are far too many variables at work for a meaningful answer to be provided. Please consult with a licensed Engineer in your area to have this evaluated." And if you feel compelled to comment upon the sanity of the project at the same time, then, well, that's what comments are sometimes used for.
It's probably worth pointing out that questions that are closed are likely going to be deleted. I think our community is better off focusing on answering questions that are more likely to be kept around and add value to our site.
It's probably worth noting that 2nd and 3rd examples provided are a little different than the first. The roof structure and large aquarium questions were more about evaluating what was already there as opposed to modifying the integrity of the structure.
And that also brings up a related comment from Mahendra:
I think if this question in the form "What are the necessary structural engineering parameters that need to be considering when removing the supporting post?" IMHO This type of question is valuable for this site
We need to walk a careful path here. On the one hand, the suggestion represents an Atwood Transform as mentioned in the article Q&A is hard. But we need to make sure that the transformed question remains reasonably scoped. With the remove support posts question, I'm not convinced that an Atwood Transform would work on that question. I think the resulting question would remain too broad and not a good fit for the site.