Wikis rewritten. Specific examples and descriptions provided, even. Also came up with ad-hoc 'categories' ("types") to better organize the examples. Language may not ideal and have a sufficiently neutral tone for wiki purposes (because I had to work on them on short notice), so someone should go over them.
Anyhow, as previously mentioned, these are two distinct concepts that describe visually different things. And, unlike, say, the leaf fan/uchiwa pair which was frequently 'misused' (IMHO), people who tag one tend not to tag the other, as can be seen in robot_ears -mechanical_ears or mechanical_ears -robot_ears searches. People know what they want to tag, and what they want to search. So, really, unlike most other 'confusing' examples up for aliasing, barely any tag gardening is needed for disambiguation anyway.
To further clarify: the robot_ears is used to describe a common visual design element found on android and humanoid robot characters. Especially on 90s/2000s manga/anime, where it is used as a visual shorthand for "I am a robot", that the audience can tell at a glance even if the character is barely visible. They often do not appear to resemble actual biological ears, but instead take the form of headpieces that look like headphones or headsets. They are only called 'ears' because they are perceived to be the android analogue of human ears, as they are 'mounted' in a similar location and are implied to hold sensors.
Meanwhile the mechanical_* line of tags describe mechanical appendages that imitate their biological counterparts. So mechanical legs should look like legs, mechanical arms should look like arms, mechanical tail should look like tails, and... mechanical ears should look like ears, and not... HEADPHONES.
(And there's a whole subcategory of robot_ears that are basically just "a specific variant of headphones-looking robot ears". Which we should probably consider an implication for, now that I think about it.)
Furthermore, while robot_ears often imply that their 'wearers' are, well, robots (or at least someone trying to cosplay as one or give off a 'robotic' feel), the mechanical_* line does not, as those can often be prostheses or equipment instead.
(This is also why we call the tag robot joints "robot_joints" instead of "mechanical_joints". It's a related tag, one also used as a shorthand for, again, "This character is a robot", though it is far less visible than robot_ears.)
Though again, I do acknowledge that the potential for confusion does exist, so maybe we should rename the robot ears tag to something like robot_earpiece instead.
Edit: One last thing: Unlike the leaf fan/uchiwa pair (which I only care because I am a pedant), more people are likely to care about the robot_ears, mechanical ears distinction. Current vote on the alias is a 50/50 split, with many senior users against (and also for), and personally I think most people would want a tag just for Multi/Chachamaru-style ears. If we force the alias through, someone is going to object to it in the future and try to raise another forum thread to suggest a reversal, or to come up with their own tag to retag this (which I highly suspect will be robot_earpiece or some variant).