1. There is no requirement as to how extensive edits must be (and rightly so)
We have very few requirements that editors need adhere to. If you are editing and you only want to improve the question, you are allowed and encouraged to do so. Just like you are allowed and encouraged to make an improving edit to a post that doesn't solve all of the issues that post might have. There's no reason that an editor would have to add links and citations to answers if they do the question or vice-versa. Of course, it is always better if you make all the improvements that you can see on a post while you are there, but in no way should we be making it a hard rule.
Make improvements to a post if you can and to the limit of your ability and energy. As long as an edit makes a significant improvement to the material it is a good edit.
2. Either reference is fine, both are better
Either paper or online reference is fine (and certainly better than none), but having both is the best option if you can manage it. This applies equally to writing as to editing an answer. See this Q&A for a more in-depth discussion.
3. "Assuming" the source is something we already do and is fine in cases where it clearly isn't the issue
"Assuming" the source is fine as long as you are certain that the source doesn't matter (eg is not the source of their confusion). After all, if the question is referencing a certain thing people are naturally going to assume it comes from a standard source even in the absence of an explicit citation. Adding a source in that case does not hurt anything as long as you are certain that it is not a factor. As always, editors must use their best judgement with their edits, and harmful edits can be reverted.
As far as commenting goes: if you feel like the user is constantly under-citing their questions or answers in a way that severely negatively affects their content, make an edit and leave a comment telling them that you would recommend improving that aspect of their post-making in the future. However, leaving comments just because a question didn't link to DnDB on a spell that they quoted the relevant parts on and just didn't cite the page # (or URL) is likely at least overkill if not overly aggressive.
We have the power to edit to improve posts for a reason, and if the way a user is making their posts is not negatively affecting their posts, there is no need for them to correct anything.
4. Links to every name in a post is not necessary at all, but also not harmful
Again, we don't need to do anything here. The site will certainly not burn to the ground for the lack of a hyperlinked spell name and we already have a Q&A talking about the value of adding such things. If the edit makes the post better, it is a good and acceptable edit.
Adding a link to a feature name can certainly be at least a slight improvement and is generally not harmful (again in cases where the source of the content from OP is wrong or inaccurate is about the only time where this is the case). This is a case where we likely can't have a solid rule about when is good or bad to add this, and should be left up to the judgement of the editors. The vast majority of the time, the edits are at least benign, so making rules about when they can be made seems completely unnecessary.
Most of the time the only issue is when these kinds of edits are made in rapid succession, but that issue is being handled in a separate Q&A.