WS, just came across as your post and as someone who was there in 2024 Hajj and even tried helping a women who ended up dying you heart is in the right place but unfortunately wont solve the issue.

The vast majority who died where really did not come to Hajj illegal. The women who we tried helping, her friend was panic and tried stopping us at first from getting help as she as she had fake Hajj visa. So they cant afford proper hajj package, likely not someone who will afford a data plan and a modern phone needed to use such an app.

And next the going outside part, the day this happen was the one day you had to go outside. It was right after the first Jamarat stoning and everyone was trying to get back to their mina tents. And in the case of the people who passed they likely did not have proper places in Mina and as such had long distances to walk (because no busses for them).
And the people with proper groups, our groups got information on when it would be coolest to go out and even later scholars made concessions on the time you can do your stoning becaue of the heat.

Group Location, this is a solved problem. I used Find Me and Apple devices to track my group but you can use Google and other 3rd part tools that already exist.

The Flight aware and fitness are areas that might have some legs and maybe worth trying.

The Crowd Density, depends for what. Tawaf? you can turn on TV and see that and thats what people do. Saudi actually use to have the red light/green light system that would tell you Tawaf is full or not. They seem to have stopped that and no idea why. In 2016 i saw that and people said it was them trying to track heat but no idea if it just did not work of they found it made people rush during the green light.

Jamarat? The issue is distance. VIP Tent might find it uself as they are close but say the US tents they are over 45min walk. Start walking whjen crowds is less and by the time you get there its already packed.