Well, one way could be that the protagonist unlocks a new level of understanding about the Imagine Breaker's mechanics and uses that knowledge to upgrade it. Another possibility is that they receive guidance from a mysterious mentor who shows them how to enhance its capabilities.
Partly true. 'Breaker Morant' takes inspiration from actual happenings, but there could be some fictional elements added to make it more dramatic and engaging for the audience.
Yes, the Wind Breaker manga is over. It's been a wild ride from start to finish. You'll find that the ending leaves a lasting impression and gives a satisfying conclusion to the characters' journeys.