Wait, is it actually the canvas that's doing the work, or is it the breathing thing? Because a tarp that traps moisture is just as bad as one that leaks, and that might be what killed your bottom third more than the rain did. Plastic tarps don't let anything out, so every bit of moisture in that wood plus any ground dampness just sits in there sweating all winter. Canvas lets it escape, which is why you're not seeing mold now. Your west wall spot is still getting hammered by afternoon sun and driving rain though, so you might want to leave a gap between the stack and the wall for air to move through.

Used to roll my eyes at the "we only read the safe stuff" complaint, figured people just want what they want. Then my own club went six months without a single book that made anyone actually disagree, and I got it.