I am trying to fix a Huffy Blackwater bicycle (15 speed). The rear derailleur is missing. Can I use a 6 speed derailleur from a Next bike? Can I disable the indexing of the Huffy (Falcon) shifter? Thanks

You can use the 6 speed rear derailleur with no problems. The 5-spd shifter does not need to be disabled, as it will work just fine.

The shifter controls the rear derailleur, telling it how far to move. The rear derailluer just follows what is dictated, so there is no real difference mechanically between the 5spd and 6spd derailleurs, only in the shifter.

If you wanted to make the bike 6spd, you would have to change the shifter and gears along with the derailleur.