>>282869516
He doesn't have to stay in zetsu when parallel future is active. He lost his zetsu the very first time he used it, but the 10-second illusion remained. Staying in zetsu permits him to activate parallel future multiple times in a row, but he doesn't have to stay in zetsu to maintain it after activation.
I don't think there's anything stopping him from using Ren to pummel people in the ten seconds they're unaware.