When mountain is created using plate tectonic like Himalayas it would have higher density, but that density is also limited, but if mountain is created using volcanic action the density of that mountain is low, under sea mountain ranges near Philippines, Malaysia and Indonesia would have less density compared to a normal sedimentary rock from a normal earth region.
Let6s bring this back down to earth again before you start on dark matter. Are you saying that sedimentary rocks are denser than volcanic rocks? That simply is not true as a generalisation. If you are basing your whole anomaly argument on one fact that is not correct then you do not have a logical case.
Sedimentary rocks are mostly quartz and clays, their density is determioned more by their packing than by theuir chemical composition. In general expect 1.8 - 2.4g/cm3
for most sedimentary rocks.
Igneous and metamorphic rocks will normally have densities above 2.5 and be determined more by their chemical composition rather than their packing. Normally expect 2.5 - 2.8g/cm3
but igneous rocks high in iron, lead, etc (pyrite, galena minerals.....) densities will push up to 5 and above. While these can be found in sedimentary rocks, by their very nature many will have been weathered, oxidised and washed away, so again density is more about particle packing and sedimentary rocks are in general more porous = lower density.
Another point you make is about mountains being under water and gravity being measured at the surface. While the average depth of the oceans globally is about 4km, the radius of the earth is about 6300km, so thats only 0.063% of the earths radius. Not enough to be of much significance.
You should add a further option to your poll: Gravitational anomaly - there isn't one.