Glad you're asking, Vargr, about the dot sign because. I don't really understand it either.

Here is what I *THINK" I understand, and I would be happy if experienced Bashers could comment on one or the other:
I'll try and answer both your questions in an understandable way... let me know if what I say is not clear enough.
1. If a mod has only one single tag, it is safe to deactivate it after import. When you deactivate it, it will show the dot then and the mod content will be play correctly in-game.
pretty much 100% dependent on what the contents of the esp is - for example: if all it does is change graphics of exisitng items this is the case, however it it changes graphics and adds new items you would get the graphics changes but not the new items if it was deactivated... you will only get what you import.
2. If a mod has multiple tags, but you do not import them and still deactivate the mod afterwards, the dot will show up and the tag field is green. But the mod will not work entirely correct. I would believe because you did not import ALL the tags.
same thing really - if you import all that you want/need from the file it will be fine but you can't import all types of changes (ie new items, some changes to existing items) so not always the case.
3. On what criteria do modders base their choice of tags? I mean since they are applied manually using common sense (I believe), a tag could be forgotten or a tag could be assigned unnecessarily. Possibly this could produce issues one way or the other after import into Bash Patch?
that is true (as happened on the initial release of MMM - Knights.esp) however it is easy to fix (by the user or next release)... of course it can cause issues (f.e. the MMM Knights was missing delev tag which meant the LLs weren't getting built quite as intended)
4. What does merge do or can do that import does not?
merging can merge more things such as NPC stats... however it is broadspectrum; if it changes the stats and the graphics you will get both whereas if you tag it just stats and don't merge it you will get the stats and not the graphics (exampliary description)... merge is also totally limited by LO, whereas import is only affected by later loading mods with the same tag and that are also being included in the patch. Import is good for things that just want to override one part of record or that really have to take effect.
5. Theoretically, one could build the mega-uber-super Bash Patch and deactivate the whole load order (kind of)? What speaks against that other than taking a lot of time and making tracing bugs and incompatibilities impossible? But for a mature, stable installation - theoretically - this might be very economical?
Sorry if the above is all a bit incoherent. But these are just some thoughts about WB. NO TRASHING OF WB INTENDED AT ALL! I LOVE IT AND USE IT CONSTANTLY! Really just trying to get a better understanding of the concepts.
the problem would be that Bash patch doesn't have the programming to include the first definitionof a record in the patch... just changes to said record (which could be just changed that Bash does)... and the dangers of letting it to that might be more than its worth for most users. and for worldspace or dialogue editing mods that would be impossible right now... at least for my coding.
(hope those answers also answer you, Vargr... if not just ask for more info.
Pacific Morrowind