This is something that has bothered me about their teaching since DAO. However, the belief is that when they remember what it is to be true elves their gods will return to them. It is not through prayers but through earning their recognition by their actions and behaviour. This seemed odd considering they also taught that these gods had been shut away and were unable to act. What I now believe is that we have some strands of misremembered history here and to some extent they had remembered it back to front, so when their gods return to them, they will remember what it is to be true elves. They also believed the Fade was a holy place, once home to their gods and that Fen'Harel continued to prowl the Fade, feasting on souls of the dead and guarding the prison of the gods, which was the Eternal City at its heart. This would then fit with what we now know about the Veil and the effect it had on the ancient elves.
Clearly the actions of Fen'Harel were devastating on the elves but may be even the most ardent followers of the Evanuris had to acknowledge that they couldn't restore their gods without him. Killing Fen'Harel would have left their gods in limbo forever. Also he must have left his own followers to act on his behalf and to guard his body whilst he was in uthenera. He may even have promised that once he recovered his power he would put things back to what they had been before. So down the years these various strands of belief were handed on. The majority of elves that were enslaved by Tevinter came from Arlathan Forest. We still do not know the exact nature of this community. Given its extreme hostility to intruders it would seem it was a remnant of the civilisation, much as the Temple of Mythal in the Arbor Wilds. Since the Dalish had such a benign view of the Creators, it is likely that the Arlathan community were descendants of the supporters of the Evanuris. Likely there were elders who slept in uthenera, plus those born after the Veil was raised who tended to their needs. For all we know, these elders were slumbering to build up their strength for push to restore their gods, charging up their foci for use in destroying the Veil and freeing their gods. Tevinter destroying the place may have had the bonus of thwarting these plans. At some point foci came into the possession of ancient Magisters because Dorian saw pictures of them in the archives. Personally I'd put their acquisition much further back, shortly after the raising of the Veil rather than at the destruction of Arlathan, since it was meant to have been submerged, making recovery of artefacts somewhat difficult, but the possibility is there.
The Dalish fixation with recovering their lore and culture probably does stem from the inhabitants of Arlathan being aware of the need for ancient magical expertise if they were ever going to get their gods back. If they remembered what it was to be true elves, they would likely have that expertise and the knowledge of what they needed to do, so their belief system isn't quite as illogical as it first appears. In any case, it is clear that the vagueness of Dalish knowledge of the past is more about the writers wanting to keep things hidden rather than a coherent set of beliefs. Why don't the Dalish know more about their history than they do? Because the writers chose to make it that way. It is odd how we can stumble around the Dales for a few years and uncover no end of ancient ruins, artefacts and pieces of lore, yet the Dalish were actually there for some 250 years without finding out any of it for themselves.
Which brings me on to a question of my own. Why were the ancient elves of the Arbor Wilds so hostile towards the modern elves? Our Dalish Inquisitor is quite right, they could have learned so much from them. They might not consider them their People but who do they think they descended from? Why were Abelas and co simply guarding a Well that they never intended letting anyone use and then suddenly decide to relinquish it because we "played nice". What was their purpose in remaining there? One of the ancient texts that we can only read if we drink from the Well says: "We must prepare for those who cast Mythal down." I originally thought this related to an earlier period but what if they also had the shared belief that eventually the Evanuris would return. Mind you, I thought it odd that the Creators would still be honoured in her temple if they had been responsible for her death, so once again the writers were withholding information from us for plot purposes. I also found it annoying that we were never allowed to ask Abelas the pertinent question: "Who killed her?" Even if he had replied: "I don't know", that would be something but he knows it wasn't Fen'Harel, so how does he know? Then after millennia of guarding the Well, the Temple and even the surrounding forest from intruders, they simply up and leave. Makes no sense to me unless of course Abelas really did recognise who Solas was. In which case that last exchange between them was likely a coded message: the time has come, the ones who cast Mythal down are about to return. (Bear in mind that at this point Solas didn't know his orb was going to be destroyed in the final battle with Corypheus and if it hadn't he would have proceeded with his original plan).
Of course the above ideas only work if Solas was in your party and you did the rituals that prevented hostilities. I've no idea why Abelas should have relinquished the Well and left the Temple if Solas was not there to encourage him to do so. Also, apart from one reference in the Temple and another in JoH, the Forgotten Ones are pretty much overlooked. They were the Dalish bad guys and said to be pally with Fen'Harel but ultimately betrayed by him as well, yet the reference said they dwelt in the Void and you seemingly had to go there to find them. Presumably that is where they are locked away but they did not regard themselves as the same as the Creators and thus presumably were not seen as Evanuris. The Evanuris didn't see Fen'Harel as a friend (the Dalish certainly got that wrong, only Mythal seems to have been friends with him), were clearly at odds with the Forgotten Ones, but where did the story come from that Fen'Harel was trusted by both and able to act as mediator between them?