Yes, you can upgrade the Leviathan Axe before beating the story. There are certain areas in the game world where you can find the necessary resources and blacksmiths or upgrade stations to enhance the axe.
It depends on the game's design. Sometimes, specific conditions or achievements need to be met to upgrade the Leviathan Axe before the story's end. But often, there are opportunities to do so if you explore and complete certain side quests or collect necessary resources.
The information on whether the entire army would return after the attack was inconsistent. There were some articles and comments that suggested that the whole army might return, but there were also some articles that said that the possibility of returning was not high. Tianmei officials did not officially announce that the whole army would return or be permanently removed from the shelves. Therefore, it was currently uncertain whether the entire army would return.
Synonyms for returning to the martial arts world were gambling tyrants, Dragon Tiger Sect, Jingwu Sect, washing up in the golden basin, Little Li Flying Dagger, God of Gamblers, Bandit Commander, One-armed Blade, Limp Hero, Roll Earth Again, and Rise Again.
I haven't heard anything definite. Maybe there's a chance, but it could also be that the story has reached a natural conclusion. We'll have to keep an eye out for official announcements.
I'm not sure. It could go either way. Sometimes characters come back in unexpected ways in manga. Maybe there'll be a twist that brings Gojo back into the story.