The rain cover for the Lowepro Flipside Backpack 400 AW III is included, but it is no longer integrated into the base of the backpack as in previous versions. Instead, it is supplied with the rucksack as a separate accessory.

No, the belt is firmly attached to the back.

The MacBook Pro 16 (...new design with Mx processor) has room in the laptop compartment, but it gets a bit tight in the area of the zip at the top. I use the Flipside with mirrorless equipment, with a larger DSLR and larger lenses it will probably be even tighter.

It should actually fit, my Nikkor 200-500 has plenty of room. However, not too many other accessories fit in with such a lens.