Slain in the spirit: Struck by the power of God (plenty of examples in both OT and NT, such as Daniel being on his face in the presence of the angel, etc.); tongues (someone stepping out in faith to speak) without interpretation (no one else being bold enough to complete the process, meaning that God's gifts are coming through human vessels), sorry it's not as perfect as you'd like it to be; prayer language: the term isn't found in the NT, but there are passages that indicate that this is a valid concept: "For the Spirit helps our infirmities, for we know not how to pray as we ought, but the Spirit himself makes intercession for us with groanings that cannot be uttered." "I thank God I speak in tongues more than you all" But IN THE CHURCH I would rather speak five words with my understanding, to teach others, than ten thousand in an unknown tongue." " But how is it then, brethren? When you come together EVERY ONE OF YOU has a psalm, has a doctrine, HAS A TONGUE, has a revelation, has an interpretation. Let ALL things be done unto edyifying (emphasis mine).
Your objections have more to do with perceived disorder than with actual practice. In the case of tongues, there is a public and private use. You confuse the two. And, because sometimes tongues are given and no interpretation follows, you then say that it proves that it is wrong, rather than understand that we are all "Christians under construction" and as such mistakes will be made. I think you're a lot more offended by that than God is. As for private use of tongues, can you really make any sort of case that this is wrong? What harm comes from it? What damage does it do? I've been doing so for 29 years now...are you seriously going to tell me that my walk with God has been derailed or damaged because of that? No way, my friend, it has enhanced and edified, it has drawn me closer to Him, it has blessed me when there were no words I could say to pour my heart out to God. I feel for those who can't or won't accept this gift. They don't know what they're missing.
I'm curious. What purpose does praying in tongues serve? What about it makes it a manifestation of the Spirit of God?