Back when my daughter was active in her addiction, she knew several trust fund heroin addicts. One had his rent paid directly out of a trust. He died of an OD last year. I think one of them had to periodically demonstrate evidence of a clean drug test to continue to receive payments.
Trying to control outcomes from the grave is the ultimate in codependency.
Any addict worth their beans knows how to game a pee test.
That someone was in rehab does not mean they were or remain clean.
That someone attends AA/NA does not mean they are clean.
Endowing adult children with an inheritence often contributes to a sense of entitlement. There are so many worthy organizations including the Salvation Army that can put donations to good use.