Is defense more important than offense in AFL?

Locks and second rowers being very similar/the same is tbh an outdated concept. Second rowers play on the edges and locks play in the middle third. Realistically locks are more similar to props than they are second rowers and second rowers are just as similar to centres as they are locks.

Where the confusion comes in is the fact that some coaches don't really name their side like that even though it does indeed play like that, so one of the named second rowers will play tight (lock) and the named lock will actually play on the edge. This is why people still use the term "back rowers" which encompasses all of 11-13 and just denominate one "tight back rower" which should be the lock, wearing 13, but sometimes gets named at 11 or 12 anyway. Just people taking a little while to adapt really.

Traditionally halves have played first receiver (halfback) and second receiver (five-eighth) but many teams these days split their halves left and right like they do with their second rowers and their centres, so they each play first receiver on one side. The fullback stands at almost a traditional five-eighth spot on the open side, which is why the nature of the fullback position has changed so much in the modern game, making it much more like an old five-eighth in attack. Teams do generally struggle to gel without an organising half and a creative half though regardless of whether they play left/right or first/second.