fundamental components of the Dune universe and how it works. Some of your other questions I don't have answers for because they're based on artistic license that the filmmakers randomly chose when making the movie.

Nobody uses lasers or projectile weapons in the Dune universe. This is primarily because the Holtzman shields are used pretty much everywhere. Projectile weapons are basically useless because they won't penetrate a shield. So nobody has them and nobody uses them. Shields are everywhere. Every human has a personal shield device. Every warship and spaceship has a shield. Every thopter has a shield generator. Lasers are likewise never used. Lasers interact with a Holtzman field in a way that creates pseudo-atomic explosion that feeds back on the laser and kills both the person shooting the laser as well as the target in an uncontrolled atomic explosion. Because Holtzman shield are everywhere, it's suicide to use lasers in any type of combat. Consequently they aren't used.

Without being able to use projectile weapons and lasers, fighting has reverted back to old hand-to-hand sword style combat. That's the generally used method of all combat in the universe at the time of Dune.

Arrakis (Dune) is a special situation though. Holtzman field generators (shields) attract worms and drive them into an uncontrolled killing frenzy. So shields are generally not widely used on Dune because they attract worms that will kill you. There's a giant shield generator (the shield wall) that protects the major cities. The major cities are built on hard rock where worms can't go, but nobody uses a personal shield on Dune. As for the specific scenes from the movie you're referring to, that's a filmmaker's choice on how to make an exciting battle scene. In the books, Paul leads the fremen on raids against Baron Harkonnen's spice equipment by planting mines in the sand in front of them and then ambushing them in hand-to-hand combat after they're disabled.

As for the fremen, they've been on Dune for 1000s of years. They've been fighting against whatever ruling house has the fief of Arrakis for that time period (the ruling house on Dune changes every 100 years or so). They're basically loosely connected tribes who are independent. Because they're not united, they never gain any traction against the offworld overlords. Paul's function is to unite the tribes into one united force instead of 1000 separate tribes. Also, Paul teaches the fremen the "weirding way" of fighting that he learned from his Bene Gesserit mother. This is a new way of fighting that the fremen haven't been taught. The movie is condensed into a short time period. In the books, Paul takes a number of years to unite the fremen and teach them this new style of fighting. Years of training and preparation doesn't make for a very exciting movie though, so the film has plot holes to make it run together in a compact 2 1/2 window.

By the time Paul leads the fremen against the Baron and the Emperor in the books, Jessica has given birth to Paul's sister, who is a young child at the time of the final battle on the plains of Arakeen. The sister (Alia) is the one who actually kills baron Harkonnen with a gom jabbar. Paul and Chani actually have a child who is killed by the Harkonnens on a Harkonnen raid against the fremen. This whole passage of time where training occurs, children are born, and teenagers grow into adults, all of which that explains the outcome better is missing from the movie.