My thoughts, honestly? Well, to be frank, I'm a bit befuddled by some of these questions. If I didnt know better, I'd have to wonder if these questions really reflect your own thoughts/beliefs, or if you're just "throwing them out there" to see how I respond to them. But anyway...
In the interest of time, I'm going to take your post in 2 parts. I'll deal with the first part now, and try to come back later for the second part as time permits.
You say speaking in tongues refers to preaching the gospel (?). Frankly, I'm flabbergasted by that. So I'll start with a question: what scriptures do you see that indicates that "speaking in tongues" referred to preaching the gospel? Frankly, I dont believe there are any such scriputures, but I'd be interested in seeing what you provide us with on that.
But to answer your question...
Lets just go back to Jesus words in that passage in question. He said:
"17And these signs will accompany those who believe: In my name they will drive out demons; they will speak in new tongues; 18they will pick up snakes with their hands; and when they drink deadly poison, it will not hurt them at all; they will place their hands on sick people, and they will get well."
The key word there is sign. Obviously, the word "sign" is a referring to miracle, something
supernatural. The other things listed there are clearly
beyond human capability (driving out demons, healing the sick, being unharmed by poison or serpents, etc). So clearly he is referring to things that are specifically enabled by the miraculous power of God.
Now for example if I were to learn Spanish or French, and go to Mexico or France to preach the gospel, I'd be spreading the gospel in a language that I've learned, but
that would not be a miraculous sign...because I'd be speaking based on an ability I already have. If a person goes and preaches the gospel in a foreign language they know,
there is NOTHING supernatural about that. That wouldnt even make sense in the context of Jesus's words in
Mark 16. Clearly, speaking in tongues, as Jesus referred to in those verses,
would only be a sign or miracle if it were unknown to the person speaking it.
Secondly, speaking in tongues was never for preaching the gospel. There's just no scripture anywhere for that. In
Acts 2:11 it says "they were speaking the wonderful words of God" (that is, praise) but the gospel was not preached to them in tongues. After they asked "what meaneth this" that Paul preached the gospel of Christ to them. There were 15 nations/languages spoken of there (v 9-11) but paul spoke to them in a common language they all understood -- probably Greek, or maybe aramaic. Thus we see clearly that tongues served as a sign, but
it was not used for preaching.
Looking further in scripture,
where speaking in tongues is mentioned, it is never for preaching the gospel. (In the book of Acts, for example, we see incidents of people speaking in tongues when they recieved the Holy Ghost, but the idea of people speaking in tongues as a method of sharing the gospel is just not there. )
Acts 10, Cornelius's house spoke in tongues when they received the Holy Ghost, after the gospel was preached to them in their own language by Peter.
Acts 19, The former disciples spoke in tongues after the word was preached/taught to them by Paul.
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1 Cor. 14... Paul speaks of tongues as being something used by a believer in private prayer (14:2,4, 14-16), or in conjunction with the gift of interpretation (14:5-6, 26-28) to edify the church body.
1 Cor 12 and 14 are the 2 chapters that give us the NT's most detailed explanation of the use and purpose of spiritual gifts, but Paul says
nothing there about tongues being for preaching the gospel.
So this whole idea of tongues being for the preaching of the gospel is just not supported by scripture at all.