Is intentionally missing a prayer considered a sin if one does not wake up for Fajr prayer, and does that entail a sin?

There is no sin upon you, and you are not considered to have intentionally neglected the prayer if you slept before the prayer time commenced, even if you thought it would be missed, because you were not obligated to perform it at that time. It is permissible to sleep before the prayer time commences, even if it is thought that one will sleep until the end of the time. However, after the time commences, it is not permissible unless one knows they will wake up before the time expires, or they delegate someone to wake them. If one delegates a person to wake them for prayer, then the delegate must wake them if the delegator predominantly believes they will not wake up on time.