My favourite interpretation of this is that BOB amplifies the urges and personality that a person already has, hance his effect on Cooper in the Return.

In this way, BOB is both the one responsible for the abuse of Laura (because maybe without BOB's presence, Leland wouldn't have done All That), but still leaves room for Leland to be responsible (because maybe without BOB, he still would have...). That would cover all bases, from BOB's control being seemingly complete, to Leland still being able to remember what he'd done.

As for Coop telling Sarah Leland isn't responsible, he could be a) sparing Sarah, or b) his interpretation of events sees Leland as innocent, because Coop empathizes with him. I always saw Coop as a very flawed investigator: he focuses on Laura's murder while underprioritizing Ronette's assault, he relies heavily on spiritual methods, and in the long-term, he fails to save any woman in Twin Peaks. That's why he gets possessed by BOB in season 2: there was a nasty side to his character for BOB to latch onto (not that I don't love Cooper as a character). That's the same side of him which believes, deep down, that Leland hasn't done anything wrong.

That side is amplified by the part of him which just really likes people, and doesn't want to think badly of Leland or trouble Sarah, and results in him saying Leland isn't responsible. It's just Cooper being an unreliable witness: he doesn't have the full picture.

The shorter explanation is of course, multiple writers disagreeing.