Tire and rim sizing, tubeless conversion? I am looking to shave some weight off my wheels. I am currently commuting on a 26" MTB.

The ETRTO size of the rim is 559x22 and the tires are 559x54. As my commute is through the city center, I want to change to slick tires.

The question here is will the 559x42 slick tires fit on the rim?

Also, can I go tubeless with "Stan's NOTUBES" conversion kit?
Can't speak to the tubeless conversion kit, but pretty much any 559xwhatever tire should fit those rims. A given rim width can accommodate a wide range of tire widths. Seeing as how your desired tires are only 2 mm different, you'll definitely be OK.