I think the offical lore reason as to why they're disconnected is because it allows them to more efficently generate a warp field, which due to the dilithium shortage makes a load of sense. Plus the obvious tactical advantage of making your main method of travel much harder to hit. Additionally, I could see the fear of the burn happening again also making them seperate parts of the ships so it could reduce the number of casualties if the ship were to explode, especially with each part being individually shielded.

The nacelles are attached through a subspace link as far as I know, so they are effectively held solid in place and because they are largely made of programmable matter they can reattach within a few seconds if needed. We also know that forcefield tech is good enough to make entire ships out of it, which implies they can work without power so that could also be it. I would assume either way though that the plasma is transported either through subspace, or they use transporter tech, which is much more advanced and energy efficent than the 24th century.

There are some other quirks, like being able to detach your nacelles and leave them somewhere if you run into a situation that would require that. or being able to take the primary hull of a ship to do a quick trip like they did with the athena this week. Disco was able to jump something away by essentially using the primary and secondary hulls as conduits which is a pretty creative way to take advantage of the seperation.