***John's baptism was inadequate: Acts 19:1-5.***
Not if the "they" refers to those who heard John speak and not to those who heard Paul speak. After all, John got his authority from GOD himself and had the HOLY Spirit.
Besides, can you show me anywhere in the GOSPELS where Christ re-baptized any of those who had John's Baptism? Remember, John's Baptism was for REMISSION OF SIN, precisely what you say Mark 16:16 is about.
Contest, context, context.
John himself distinguished between his baptish and New Testament Bapism (See Matt 3:11). John's Baptism was "unto remission of sin," wheras Christian baptism is "for remission of sin" (Acts 2:38). Jesus, in his lifetime, baptized "unto remission of sin", just as John Did.
New Testament baptism only became possible after the death of Jesus on the cross (Rom 6:3-4). That is why Paul rebaptized those who had only received the baptism of John.