I'll be honest, given how many other problems I saw in LoK, I'm not sure I'm willing to credit them with thinking ahead that far and sticking to a plan. I can certainly see the signs, I'm just not sure I believe they were part of a deliberate character arc.
It would not surprise me if the writers stumbled backwards into Korrasami after one season of trying to push Makorra, one season of 'oh crap the fans aren't buying it, abort abort', one season of 'so I guess we're not doing a romance arc now?' and a final season of 'actually, the leading ladies do seem to have some chemistry, let's run with that'.
Mind you, I still count that as a win. It ends up in the same place, it means the writers improved along the way, and on a Doylist level it means they felt they could 'run with' a canon lesbian pairing in the end.