Alternator belt

My alternator belt disintegrated not long after being replaced. Before the second one went (but before I had time to do anything about it) I noticed that it seemed to be slightly misaligned on the tensioner pulley (see photo). I had tried to manually reposition it so it was central on the pulley, but it would just slip again after less than minute of the engine running.

The mislignment seemed to be causing premature wear at the edge of the belt (also see photo) and I can see how this could lead to the belt ultimately tearing itself apart. But why was it misaligned? Do I need to replace the tensioner pulley? The belt was also making a rhythmic squeaking noise when in use but hard to know where this is coming from (maybe the aircon pulley?)

If its the tensioner pulley causing the issue, is replacement a diy job? Do I need to lift the engine or move other stuff out of the way first?

Edited 17 November, 202517 Nov by hasam37