I'm trying to figure out what TES4Edit's rules are for what you can copy from one esp to another this way.

I built a quest with an NPC and a few other items as a separate esp and now I'd like to incorporate it into another existing esp. I could copy clothing and a couple of other items that way, but the NPC and scripts and a lot of other items don't give me the option of the second esp as a target, only the one they're coming from and "". Is this just a matter of sequencing the copies, so that anything they reference is already there, or are there more complex rules? If so, where are they documented, and is there any way I can see what's preventing the copy?

Just a guess really, but I think you are right in thinking that it won't work due to missing references.

Might even be a chicken or the egg problem, if the dependency is cyclical. IE an NPC script that references the NPC
If that is the case, you would need to disassociate the script from the NPC first in the original esp.

EDIT.
Could you not just merge the two esps? Or is this mostly just for the sake of knowing?

I was also intending to change a number of EditorIDs in the process, to conform to the naming conventions of the recipient esp. There are also a few things that would be left out, so a "piece at a time" method has some advantages over a merge.

But yes, I'd like to know for future use as well.

What method of merge would you suggest if I decide to do that and then edit the result?

Not sure what you mean by method? Which tool to use to merge with?
I'm not the best person to answer, but I've used the CS 'combine loaded plugins' tool with success. It's in the file menu.

If you mean the process. I would make a 3rd patch mod. So you load the other mod, then your quest mod, then the patch mod.
Make all the alterations you need to in the patch, then you can merge all 3 plugins into 1.

If you use TES4Gecko as the tool to merge, you can also clean the mod as well. But I would use TES4Edit to get a concise viewable comparison.