There's no way to tell exactly who Jones is without more details. He could be a soldier, a civilian, or someone with a special role during the civil war.
In this type of fanfic, the Infinity Stones could have a very different effect in the Harry Potter universe. For example, the Time Stone could be used to reverse dark spells or resurrect fallen wizards. However, the Stones' power might also be limited by the magic rules of the wizarding world. The fanfic could delve into how the characters from both universes adapt to the presence of the Stones and how they use or misuse their power.
Definitely. The Infinity Gauntlet comics came out earlier and laid the foundation for the concepts and characters seen in Infinity War. There are some differences, but the comics serve as a precursor.
Avengers Infinity War is largely based on the Infinity Gauntlet comic. Many key elements and plot points are drawn from it, but there are also some creative adaptations and additions to make it work for the cinematic universe.
Aldo might be an author himself, and these are his stories that he has written. Maybe he is a relatively unknown writer, and his works are starting to gain some attention. Or he could be a character in a series of stories written by someone else, where Aldo plays a central role, like a hero or a protagonist with unique qualities.