It isn't always possible to determine a single "correct" pronunciation for a word, especially not for a word like this that is obscure and mostly archaic. People may even develop preferences for odd pronunciations that have no obvious basis in anything (e.g. "dais" is commonly pronounced as a disyllable, and apparently has been for a while--see this book from 1869--although it comes from a French monosyllable, and there is no general rule of pronouncing "ai" in two syllables in English).
You referenced the Oxford English Dictionary in the question, but I think it actually has a good discussion of this in its entry for heel, v.1:
The spelling of the word continues to vary considerably in modern use, probably reflecting its limited currency. The spelling heel is very common only in sense 2c, where some folk-etymological association with heel v.3 seems likely.
In form hool perhaps showing an alteration by analogy with whole v. beside heal v.1
The origin of the forms hail and hale, attested in Masonic usage (compare sense 1) from the 18th cent., is unclear. The forms may either originate in regional pronunciations (compare Forms 1 β.), or be due to the influence of Older Scots graphic forms with -ei-, where the postvocalic -i- indicates vowel length (compare reveil v.).
The "long vowel" in modern English that usually corresponds to Old English "e" is /iː/, but as mentioned in the quoted passage, regional variation has existed.