All it says in the DS9 companion is that Hilary Bader's original pitch was to send a red-shirt down to be doomed and the staff decided they should pick a recurring character instead and someone came up with Opaka.
Yeah that's what I've read and understand about the situation. It was interesting on the one hand that they were willing to 'kill' the character, particularly as it was done in way that hadn't been done before. It upped the stakes, really. This was only Opaka's second episode though, so the emotional resonance fell a little flat for me. I also thought it was very arbitrary way of killing off the key Bajoran religious figure of the time.
And I loved Camille Saviola in the role.
By the end of the season, though, we had Vedek Winn, which was a marvelous direction for the show to go in. In the second season, the writers pulled another u-turn with Vedek Bareil. I remember reading in that case that the writers were planning to install him as Kai, but agreed that it would be no fun having someone who is a friend in that position. So maybe
, they had that thought in mind when they were writing Battle Lines too.
Anyway. Despite the killing of Opaka being a bit glib, the episode is alright. I agree that it does have that planet of the week vibe that makes me think this could have been done on TNG, but would they have killed a supporting character off? It's not an amazing episode, but it holds well against its season one competitors.