Both are true.
That's simply false.
Yes, both are true if taken on the face value but one is wrong when it comes to the intended use/function; Gifts are to profit all and the specific function of tongues was to be a sign to the unbelievers. If a tongue fails in it's function then self edification is achieved (true but wrong) - because if i understand the gospel and fail to transfer my understanding to the unbeliever in a language they understand, then my understanding is unfruitful. This is self edification because the understanding remains with me and it is very wrong. This is what Paul says uttering mysteries- they are mysteries to the listener because the listener doesn't understand a thing.
1 Cor 14:9 Paul recommends speaking intelligible words with your tongue (not interpreting). I don't know how you are going to pull that because your idea of tongues is just words with no meaning.