I was wondering: if they had not committed suicide themselves, what sort of legal liability (criminal or civil) would Jones and Applewhite have faced for their role in the suicides?
It's impractical to say what the law of Guyana was in 1978, but we can substitute criminal law from 1998 and assume that there would be substantial overlap. Suicide is still a crime, as is murder and while many people committed suicide, many also died at at hands of others. Whether or not Jones committed any murders, he was an access before the fact, and can be treated as a principal (Title 3), and for conspiracy to commit a felony (murder). Under §96, conseling or aiding in suicide is a felony punishable by life imprisonment.
The Heaven's Gate suicides took place in California, where suicide is not a crime. However, it is a felony to aid, advise, or encourage another to commit suicide. As a backup, following the model of Michelle Carter's conviction (appeal denied Mass Supreme Court, petition to SCOTUS, cert denied), he might be charged with involuntary manslaughter, although prosecutors would have had to think of that angle (encouraging suicide is not a crime in Massachussetts, thus the need to be creative in prosecution).