Whether Leliana or Cassandra or some other person would be a good inquisitor is irrelevant if your inquisitor would leave because somebody would have to be leading the inquisitor if they want to continue it's existence.
That's kinda obvious, isn't it? But that wasn't the issue we discussed about - the issue is whether there's anybody suitable to replace previous Inquisitor in case he/she would find themselves in a situation, in which they'd have to choose whether they stay with Inquisition, or - say - embark on new adventure (btw. I assume that the DLC would be substantial and won't focus on romance TOO much, considering that not everyone chose to play female elf or romance Solas. There HAS to be some sort of option for, say, befriended Inquisitors as well - or all of them, whether approved or not, considering that they still wield the Anchor).
Solas is capable of understanding your reasons of doing something if you explain it to him even if he would have disapproved the decision at first like when you drank from the well. But because it's not known what situation it will be if you have an opportunity to join him he might not even disapprove it. Maybe he wants you to join him.
I didn't say that he'd disapprove of any choice, in case they'd find themselves in a situation that would require Inquisitor's assistance. I simply said that he'd disapprove if Lavellan made a choice based predominantly on picking ancient elves over humans or something like that.
Also - whether he'd want it or not may be irrelevant. He does leave her after all and claims that he can't be with her - what's the reason for it we don't yet know, but it may encompass everything: from him not wanting to see her eventually wither a die, to not waning to risk her life or maybe her seeing him do something terrible... and might as well be all those things 
I think Solas picked Skyhold because he thought that it would be suitable for the inquisition and I think that it has nothing to do with steering suspicions away from the elves. He needed the inquisition to succeed because he needed his orb back so that he could help his people. In a way he used inquisition.
It has everything to do with steering suspicions away from the elves - he even said that, after talking with Inquisitor before finding Skyhold. Just pay attention to what he says when he takes your Inquisitor to the side, where he reveals that the orb was of elven origin.
Also: Skyhold lies in a place that was of tremendous importance to ancient elves - you'd think Solas would just give away its location, simply because it was "suitable for Inqusition"?
My elven inquisitors feel that they are outsiders in the inquisition and they don't believe in the Chantry.
How's that different to what modern elves are to ancient elves?
One of my elven inquisitor suspects that Solas isn't just a powerful mage but instead something more and she suspects that she might need all the allies in the future because there must be a good reason for him to leave her.
I think any smart and insightful Inquisitor suspects that Solas is more than he chooses to give away. In fact, the game hints at it - the approved Inquisitor (not just elven one) asks "there's more to that, isn't it?" after Solas laments over broken orb and can say to Leliana that something else must be going on for him to leave so suddenly.
I think that ancient elves would not approve Inquisitor at first but it's possible that they will learn to approve her when they see that she is at their side. It might not happen fast but there is no reason why it couldn't happen. Afterall she has the mark and she is with Solas.
There's a ton of reasons why they also wouldn't approve, aside form those I mentioned earlier. She IS an outsider and she is a mortal - that itself separates her from her ancestors, in more than one way.
Having a mark or being with Solas may not also mean much, as far as we know - I mean, what if there are fighting factions, even among the remnants? What if even ancient elves don't know that Solas is (apparently) Fen'Harel and for whatever reason he can't reveal himself as such? What if having the mark would only bring resentment, or somebody would try to kill her because of it?
As for elves not approving at first, but changing their mind after learning that she's on their side... how's that different for humans of Thedas? Wouldn't, in fact, a revered elf Inquisitor be a perfect ambassador for her people? That task alone - representing her people among other races of Thedas, might require her to stay in Inquisition, even if only on behalf of reinstating ancient elves back to Thedas.