This paste will break down two situations in which Seto 3 can activate a set Harpie's Feather Duster to destroy the player's Widespread Ruin.

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SCENARIO 1: Set Widespread Ruin turn 1 on Seto 3, he sets backrow magic, you play THTD
QUESTION 1: What are the odds of Seto 3 activating the backrow as a Harpie's Feather Duster on the next turn, thus ruining the perfect "3-turn Widespread duel"?
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In order for Seto 3 to activate the HFD, all of the following must be true:
- HFD was the first magic card in his hand (40%)
- He did not activate HFD from the hand despite having it (25%)
- He activates HFD from the field (75%)

Thus, the overall chance is 7.5% (0.4 * 0.25 * 0.75), so you're very likely to get the god duel. In practice, I have never seen the "3-turn Widespread duel" ruined by a backrow HFD, likely because the chance is so low!

However, the next scenario is a little more dangerous.

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SCENARIO 2: Use Raigeki/Dark Hole/DCJ to clear Seto 3's board, he sets backrow magic, you respond by setting WSR
QUESTION 2: What are the odds of Seto 3 activating the backrow as a Harpie's Feather Duster on the next turn, potentially causing you to take up to 4500 damage?
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The math here is more complicated, but the odds are essentially the same. However, the consequences of landing the 7.5% here are more dire, as you will take a direct hit from whatever monster Seto 3 plays. This is worth considering if you have WSR in your hand when you use Raigeki/Dark Hole/DCJ, as you could opt to toss it and draw an additional card. Although WSR is useful 92.5% of the time in the scenario above, it doesn't really help you if Seto 3 plays HFD from the hand instead of the field. It hurts you by being a dead card in your hand if Seto plays a monster after you use your Raigeki/Dark Hole/DCJ (which would prevent the scenario from happening at all).

The second scenario played out in Dark_Rit's Season 3 tournament match vs. AilsaNyx, and it worked in his favor: https://www.twitch.tv/videos/1730955478?t=01h54m57s