Just clarity cleans up the muddy sound. Not tone. The magnet changes the output of sound.

Yes. You would have less magnetic pull on the strings. I put an alnico 4 in a Super Distortion (usually ceramic). I had to use hot glue to keep the magnet from moving around though. Some pickups intended for larger magnets or multiple ones, will need some kind of glue or a spacer/shim to keep it from moving around. But changing from ceramic to alnico sounded like a louder, more compressed PAF. No midrange "onk" or excessive spiky treble. It basically fixed the pickup. Highly recommend Alnico 4 first, before any other magnet. It tends to be more "even" in frequency response, so if there's an annoying aspect of your pickup, usually A4 fixes it.

A fully charged magnet should last "forever" if you don't put your guitar near a powerful electromagnet or an MRI machine or something. Magnets can be "demagnetized" but only a significantly more powerful magnet can do that. However, vintage pickups typically had magnets that were not fully charged, due to production process reasons and they are unstable and can be more easily de-magnetized. People have placed old guitars next to a guitar cabinet and the pickup discharged and left the pickup dead. But that was extremely rare.

Watch the video and he tells you.