What would Tywin have done?

Tywin would have never send Jaime to Dorne
He would have never befriend High Sparrow
He would have never give them arms

Basically, he would do the opposite of what Cersei has done

If Tywin would still be alive, we wouldn't be asking these questions.

We would be asking what is a High Sparrow.

Except the Queen of Thrones was a crusty and snappy so she should have done the same as Tywin.

I think even Tywin would have had problems with the smallfolk eventually. Their anger was mounting, and he could be pretty tone deaf. Tywin ravaged the Riverlands, sacked Kings Landing, had the baby prince and princess and queen murdered, and empowered men like the Mountain to rape and pillage. Anger over those atrocities probably would have come home to roost eventually. Tywin also allowed a lot of rottenness in his administration. The High Septon was rotten, the Grand Maester was rotten, nobody liked Varys or Littlefinger, the boy king was a bastard, and so on. One legacy of Tywin's rule was that the smallfolk loathed the Lannisters.

Olenna and Tywin both signed off on Joffrey and Margaery's ostentatious wedding at a time of austerity and suffering. In the books, Tywin wanted to send Tommen back to Casterly Rock and rule in his stead, which seemed unlikely to go over well.

If Tywin had lived long enough, there would have been a smallfolk rebellion against him. Tywin's instinct would have been to crush it mercilessly. And that could well have backfired into a full on peasant revolt. The smallfolk hated him and had very little to lose, especially with winter coming.