LF Draenei didn't get their shaman class because they were never on Draenor to learn from the Orcs.
You have to understand that Blood Elves and High Elves aren't actually too different from each other. They split on a fundamental level, High Elves did not agree with the way Kael was leading his people, and then later with Lor'Themar siding with the Horde, which no doubt left the High Elves that remained with the Alliance bitter for their home land to side with the people they fought no long ago.
Their split wouldn't change the way they use certain magics, especially the Light, as I explain below.
I think making them similar would actually be better to show the struggle between High vs Blood, to show that they're indeed the same people fighting a civil war. Having them be different would make the High Elves just Void Elves, which would be honestly be a missed opportunity to build on the race's struggle. Talking about Void Elves, I doubt High Elves fundamentalists would look to keenly on Void Elves either.
High/Blood Elves never needed the Sunwell for their holy powers. It was their faith in the Light that they gained holy powers, the same way as Humans. And Blood Elves lost theirs because they lost their faith in the Light after the Third War. If they don't allow High Elves to be Light users they'd better have a really good explanation to why High Elves would voluntarily give up their holy powers, considering they've been part of the Church of the Holy Light for such a long time.Could they become paladins and priests? I believe they could, yes, because they were once. Perhaps they voluntarily gave up the holy classes because of the sunwell, or were they cut off? i honestly can't see yet the best solution to this.
Should there be a difference, yeah sure. Not allowing them to be certain classes like Warlocks works within the lore. But to not allow them to be Priests and Paladins for the sake of having a difference is highly unnecessary, and takes away the special relationship they have with the Blood Elves.