Different Gifts?
There is no evidence that tongues spoken in private and the tongues spoken and interpreted differ in nature or are two different workings of the Spirit according to this author. It is the same gift, used in different ways. Although many do contend for this I do not believe that there is enough scriptural support for such a view.
Tongues without interpretation is evil
Tongues that are without interpretation do not immediately negate the tongues from being a legitimate manifestation/working of the Holy Spirit. What marks a false tongue would be the flesh and evil spirits. A tongue is evil when it is sourced from an evil spirit, not when it isn’t interpreted.
The gift of tongues marks those who are truly saved
This is not correct. The gift of tongues are available to those who are truly saved (Born of the Spirit of God).
Tongues are always from the Holy Spirit
A illegitimate gift of tongues can also manifest in extreme cases of union with evil spirits. I have however not encountered this that I can remember of, although I have heard of demons praying through people during deliverance sessions.