QWhen my Oshi holomem is hBP04-006〈Oozora Subaru〉and my center holomem as well as 1 of my back holomem are hBP04-067〈Oozora Subaru〉, my opponent used hBP01-087〈Kobo Kanaeru〉's Arts "Rain Mantra".
If I use the Oshi skill "Mental! Physical! Passion!" when my back holomem hBP04-067〈Oozora Subaru〉is dealt 20 damage by the ability, can I -30 damage from the 40 damage dealt to my center holomem hBP04-067〈Oozora Subaru〉after that?

ANo, it cannot.
As the ability can only reduce 1 instance of damage, it can only reduce the damage at the timing that the Oshi skill is used.

QWhen 2 of my 〈Inugami Korone〉 were downed by the ability of Arts "Rain Mantra" of the opponent's hBP01-087〈Kobo Kanaeru〉, can I use the SP Oshi skill "Wow Wow Wow Wow" of hBP03-006〈Inugami Korone〉 to reattach the cheers from both holomem and return holomem to my hand?

ANo. As down processing resolves for 1 holomem at a time, you may only use it for either 1 of the downed holomem.