Ths derivation from Welsh seems unlikely (Cornish even more so). No parishes existed when Welsh was last spoken here, the Celtic church worked from monasteries. Welsh for parish stone wd be carreg plwyf -maen plwyf is unlikely, plwyf maen is very unlikely as adjectives generally follow the noun.
Most likely to derive from old anglo-Danish Bla Man which means Grey/Lead Coloured stone. Supported by Blue Beck and Blawath Beck close by-it wd make more sense for these to mean the grey becks rather than parish becks. Modern Danish still has the word Bly for lead. Grey Stone is exactly what the Blue Man is. Man unlikely to derive fm maen , English has hardly any old words derived fm Welsh, man is widely used for prominent cairns/standing stones. Found in other languages too eg Gaelic-the Fhir Duibhe on Ben Eighe means The Black Men these are prominent pinnacles on the summit.
Yes. These stones are all glacial erratics, mainly granitic rocks from Scandinavia. There is a large exhibition of them at the Hunebedcentrum in Borger, and nearly all are rounded: presumably the result of the grinding effect of the ice.
Guess it's the coastal proximity. I've slept a few times on The Mound and 'wondered'. Duly noted should I pass again. Hopefully. In return would recommend the stuff around Tain, if you haven't already. Kinrive etc.