yes it works.

no, it isn't as reliable as a new chain or a master link unless you are VERY careful about how well you align the pins.

yes you can probably join a shimano and a sram chain remnant together.

a new cheap chain is around $10 and even a nice one is not much more. just buy a new chain.

FWIW, I use chain remnants to make keychains, and I give them to friends!