It is not 'about to come,' for the act of the Lord's coming is not immediately before the mind. I will not say there is no allusion to the future awaited exercise of divine power; for in Daniel, not only is the Son of man brought before the Ancient of days, but the Ancient of days comes. In the words 'who is, and was, and is to come,' there is surely reference to the abiding nature of his being. 'Who is to come' seems to me thus to render more truly the sense than any other form of words. The Greek has the future sense. See Mark 10.30, Luke 18.30; and Mark 11.10 partly runs into this sense. 'The coming one' became a name of the expected Messiah, as in Matt. 11.3, and in Heb. 10.37. But note, 'essential being' is put first, 'who is;' and then 'time past, and to come.' 'Who is' asserts the essential nature of his being.
Alpha and Omega are the first and last letters of the Greek alphabet.
'Jehovah Elohim,' as Luke 1.32; see chs. 4.8; 15.3; 18.8; 21.22; 22.5,6.
'The Almighty' always has the article in Greek in the Revelation.