Ouch! Sounds like Pinky is going to be giving you a LOT of trouble! I’m guessing that the poop and pee were because he smelled Muffin, and the cage biting was because he wanted more time outside. You said he was neutered about two weeks ago? It takes around a month for hormones to officially drain out, so hopefully, he will settle down then.
You did everything I would have done, covering his pen with a towel, I don’t see why that didn’t work though. Maybe you could try extending his pen just an ft more, and cover the rest? See if that does anything? I have to say, this is quite the intense poo and pee wars, with mine, it was just ‘oh, there’s pee, better go pee on it’ not ‘I need to patrol my area and guard it against ANY intruders’ well, a bit of that too, somedays I was washing the entire pen, for SOMEONE (I’m looking at you, Silver) peed all over their x-pen. As I said though, I have my fingers crossed that this all drains out of him in a few weeks.
Keep us posted!