Why Did InGen Pick An Island With A Volcano To Build Their Park?
There was never any mention of Jurassic Park being built on an island with a volcano; this was a retcon by Jurassic World: Fallen Kingdom to create an immediate need for the dinosaurs to be transferred from the island into the mainland to fuel the story. Going backwards, though, it makes little sense; InGen would have done geological surveys about Isla Nublar - after all, they were investing untold billions into cloning dinosaurs and then building a theme park attraction. Furthermore, when Masrani Global bought InGen to actually open Jurassic World to the public, they should also have surveyed the island to determine if the volcano, Mt. Sibo, was a threat not just to their investment but also to the thousands of visitors to the park.
Whatever Happened To The Second Island With Dinosaurs, Isla Sorna?
The Jurassic World films have completely ignored that there is a second island, Isla Sorna (a.k.a. Site B), which was where the dinosaurs were cloned. Introduced in The Lost World: Jurassic Park, Isla Sorna is a neighboring island to Isla Nublar and contained even more dinosaurs, including multiple T-Rexes and Velociraptors. The drama of Isla Nublar's volcanic destruction and the imminent need for evacuating the dinosaurs is undercut by the existence of that second island also full of dinosaurs. The Jurassic World movies never mentioned Site B, suggesting they're just trying to ignore it, at least for now.
Why Did The Mercenaries Think The Mosasaurus Was Dead?
The opening sequence of Jurassic World: Fallen Kingdom is as visually resplendent as it is nonsensical: Mercenaries hired by Mills venture to the island with a submersible vehicle to extract a bone from the carcass of Indominus Rex, which lies at the bottom of the lagoon after it was killed by the Mosasaurus at the end of Jurassic World. However, one merc in the submarine tells his frightened partner that the Mosasaurus "must be dead by now." Why would they think that? Only a few years had passed since Jurassic World fell and the dinosaurs were all over the news and were the subject of Congressional hearings. Even if they were a separate group of mercs and not part of the team led by Ken Wheatley (Ted Levine), how could those mercenaries think the dinosaurs on Isla Nublar were already dead?