Do you realise how hypocritical your stance is? On the one hand, you are arguing how Oda isn't calculating how many meters Zoro's swing would be launching Pica upwards but on the other hand, you empathized on Pica's width not being the same like ordinary mountain or how busting a mountain is more impressive than cutting it in half and lifting it. Do you think Oda microanalyzed the details? Nope, this scene was meant to show "Zoro's able to cut a walking ass mountain", nothing more.
To make things even more ridiculous, Zoro started cutting the town sized arm in several pieces with NAMELESS slashes. This Zoro is significantly weaker than the one fighting Kaido or King.
Furthermore, if you believe Pica's width takes a crucial part into this debate, I can quickly post some calculations whether the force to cut and lift this slim mountain is truly that weak or not. But hey - you wanted to say Oda doesn't give a fuck about numbers and mathematics, right?
1.) The "one minute" statement was an exaggeration. As a matter of fact, what I meant to describe is how Amaterasu wasn't portrayed to be so super hot that it completely melted or vaporized Ay's arm immediately.
2.) Yes, so Ay had a moment to launch another attack but was stopped by Gaara. Then Ay decided to cut off his arm to stop the black flames from spreading onto his body completely.
3.) Nagato, with his frail-body, was completely engulfed in the flames and some moments happened before he tossed the black flames aside with Shinra Tensei - taking some damage in process.