Both Isis and Seth's reconstructed ancient Egyptian names are in Wikipedia using scholar's approaches to reconstruct vowels based on Coptic, proto-semitic, Akkadian/canaaite tablets writing Egyptian words in their alphabets and some Meroitic and Nubian vocabulary borrowed from Egyptian.
Both Loprieno, Antonio, who wrote Ancient Egyptian: A Linguistic Introduction, and James P. Allen who recently wrote a similar book talk about how they reconstructed some of the vowels. It's hypothetical, but it makes sense at least to me if you really dissect the research they've put in.
Coptic 'H' = Egyptian (long) 'u' depends on the consonants surrounding the vowel and whether it is long or short and if the syllable was stressed or not.
The two examples ?usat and sutak are perfect examples of Coptic 'H' from long 'u'.
Coptic 'H' also supposedly comes from an original Egyptian 'i'
(Fayyumic) KHNNI - to become fat = qinjit (Egyptian)
(Fayyumic and Bohairic) CHINI - physician = zijniw (Egyptian)
(Coptic) aMPH(h)E - asphalt = m_rihjat (Egyptian, **h** has dot under it)
This is why I say "originally" I don't believe Coptic 'H' had an "ae" sound if it 'originally came from Egyptian 'u', 'i' or Late Egyptian schwa and/or 'e'.
I am uncertain about Greek original words or Arabic, maybe Coptic 'H' had an "ae" sound in these words originally?! Maybe that's where it comes from?!