Thanks, Snuffy. The Milton quote is from a University of Arizona site, but no verification or source. Checking the homepage of the site, it is evident that the site is not an official Univ. Arizona webpage, but one maintained by a student or staff member.
www.u. arizona.edu/~shelton1/quotes6.html

The quote goes back before either Milton or Bayly- "Bartlett's Familiar Quotations," John Bartlett, 1855, Fourteenth Ed., 1968.

"Absence makes the heart grow fonder" : Sextus Aurelius Propertius, 54 BC-AD2, Elegies I, xxxiii, 43. Bartlett's, pp. 127-128

The second quote is "Absence makes the heart grow fonder: Isle of Beauty, fare thee well!" "Isle of Beauty," Thomas Haynes Bayly. Bartlett's, pp. 587-588.

No mention of Milton; his connection with the phrase seems to be myth, as you say.