I'm not sure if Tusk is a real story. It might have elements inspired by real life, but could also be a purely imaginative creation. It's hard to tell without more context.
Most likely not. 'The Tusk' is often a work of imagination or based on fictional elements rather than real events. Authors might draw inspiration from various sources but don't necessarily stick to true happenings.
I'm not aware of its popularity. It could be popular within a niche group of visual novel fans who are interested in the particular theme or style it offers. There are so many visual novels out there that some great ones can be overlooked while others gain a large following.
Well, 'Tusk' is purely fictional. The story was crafted from the imagination of the creators and has no roots in reality. It's all about creative storytelling and not tied to actual happenings.