It's ironic that Mercury is the chosen example of a discrepancy in an existing theory requiring a revision, because as I understand it MOND being based on Newtonian gravitation can't explain Mercury's actual orbit either. So it purports to solve one problem (galaxy scale gravitation) while re-introducing one we already solved. This makes it a bit weird that the example of Mercury is so often used in arguments for MOND.
>If you want to merge MOND with Einstein’s General Relativity, it is possible as well, simply by adding in scalar (and possibly vector) terms in addition to the standard metric tensor terms.
There's no issue as long as you think that "adding in scalar (and possibly vector) terms" is somehow a "simpler" change than "adding a new kind of matter".
But when you look at what "adding in scalar (and possibly vector) terms" actually means in GR, it's the same thing as "adding a new kind of matter". You're just calling the new kind of matter a "scalar field" (and possibly also a "vector field", i.e., two new kinds of matter).
In other words, when you take relativity into account, "MOND" is not an alternative to "dark matter"; it's just one particular way of adding "dark matter".
My understanding is that MOND doesn't kick-in in the Solar System at all; the strength of gravitation from the Sun is much greater than the MOND threshold. So I'm surprised that Mercury ever comes into discussions of MOND.
-Thanks for the correction.