I wish to make a default replacement object, not a stand-alone object. So, perhaps, I have to NOT register a GUID? I want the replacement object to be a "fix" to the original object.

yup, do not update the GUID and it will override the original.

Will the delete button be disabled or enabled if that occurs? I am afraid that the delete button will be enabled, and the player (me) has to be extremely careful not to be tempted into accidentally deleting an in-game file!

That i dont know, but you will find out soon

Most of the time you don't want to replace the entire object, just a certain part of it, so it's enough to extract that resource and modify that. If this does not include the OBJD resource, then there's no worry that the delete button will be eabled. If it does, it's better to put the mod with the game files (r.g. TSDATA/RES/Catalog/Bins) and not in Downloads.

Also, from what I've read so far, you shouldn't click the 'fix integrity' button, as that's the one making the object custom content (amongst stuff). Warning: I just started this modding business yesterday, so don't put too much faith in me. *lol*