It's important to note that a h- prefix isn't an example of lenition. While lenition originally developed as a pronunciation helper many centuries ago, it no longer serves that purpose - it has been retained in the language for other reasons entirely, so you have an bheach and an beachaire, an cailín and an chailleach, where lenition does cause a change in pronunciation, but it doesn't do so for the purposes of making "a smoother flow of sound".
In the case of cé hé (and to a lesser extent in cé hí and cé hiad), the h-prefix serves to separate the two vowels, and maintain the existence of the pronouns in speech. It can be considered a "pronunciation helper" in those two cases.
cé doesn't require a h-prefix before other vowels, as these examples from the NEID entry for "who" demonstrate:
"who's there?" - cé atá ansin?
"who that letter from?" - cé uaidh an litir sin?
"who in God's name will tell your mother?" - cé in ainm Dé a inseoidh do do mháthair é?
"who would have imagined it?" - cé a cheapfadh é?
"who else?" - cé eile?