1. Yes BJP let Congress win because they know they can easily manage congress and Rahul Gandhi even if they win few states and BJP is not dumb to alter Punjab results, they're playing smart with rigging, had they done they would've caught red handed because Punjab people were totally against SAD-BJP combine and there was strong Anti-incumbency.
2. Bihar was in Pre-Demonetisation era where bjp thought Modi Wave will win them elections so never thought of such things, but after Demonetisation Blunder which had potential to end Modi's political career, He and bjp realised in this post-Demo era only way they can win elections is to rig EVMs more specifically Gen 1 EVMs which were repeatedly used in almost all elections after DeMo. Right from Chandigarh local elections to recently concluded Delhi local elections.
3. Incidentally, in recent West Bengal polls, EC used Gen 3 EVMs with VVPAT and also Bengal elections happened amidst EVM rigging uproar from all quarters so it's foolish to think of rigging now when VVPAT will expose them. And the result was bjp faired very poorly.