The double vowels and the silent 'H's don't really make any sense in English, however there is an explanation in etymology--namely Hebrew and Greek--and transliteration of those names into English.
The Hebrew names Sarah and Hannah (anglicisation of Chanah), are English transliterations of the Hebrew characters (and I wish I had the alephbet on my computer to better illustrate what I'm trying to explain, but I don't, so please bear with me) SHIN RESH HEY (Sarah) and CHET NUN HEY (Chanah). The final character (letter) is HEY which is transliterated as the English letter 'H' and is pronounced as the English letter 'H' (almost undistinguishable when placed at the end of a word or name).
HEY is considered somewhat of a sacred characters as it appears twice in G-d's unspeakable name. HEY can also be seen as a symbol of G-d's presence within a person. (The HEY was added to Avram and Sarai's names when they accepted the law of G-d and thus became Avraham and Sarah). And finally, adding a HEY to a noun feminises it. If you take a look at English names in their Hebrew forms using the alephbet, you'll notice that most of them have a HEY at the end.
So, when names were transliterated into English, there were choices made whether to transliterate the HEY into 'H' or to drop it all together. Sarah, Chana=Hannah, Rivkah=Rebekah, Leah and Peninah are the only names that are coming to mind at the moment that kept the final 'H'.
As for double-voweled names, Aaron is an anglicisation of Aharon, the proper transliteration of ALEPH HEY RESH VAV NUN. Hebrew does not have characters for vowels, instead it relies on a series of dots that lie nearest to the consonant to convey vowel sounds. Aleph is a silent character, however because of a dot that denotes an 'ah' sound, the first syllable is 'ah'. The second character is HEY, which makes the sound of an 'H', however there's a dot attached to it, so the sound is 'hah'. Therefore, the first two syllables of the original Hebrew form of Aaron are 'ah-ha'. Apparently, the HEY was dropped, probably when the Bible was translated into Greek as I don't believe (but I could be wrong) that the Greek language didn't have a character that corresponded to the Hebrew HEY.
In the case of Isaac, I can't find a strictly Hebrew explanation, but I'm thinking it has something to do with the translation from Hebrew to Greek to English. The translated Hebrew form of Isaac is Yitzchak, the translated Greek form of Yitzchak is Isaak or Isaakios. I suppose when the name was transliterated into English, the two vowels remained. Unfortunately, I don't know anything of Greek, so I can't supply any more information here. :(
So, those silent 'H's and double vowels aren't simply a product of the English language. ;)
I hope that made some sort of sense. :)
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