Who has proven the case that Mark 16:9-20 cannot be true?
Who has proven that Mark 9-20 can be true? No one here.
In the absence of apostles and prophets who were eye witnesses it cannot be proven by external sources. Therefore we are left with internal doctrinal evidence and that is more than sufficient to prove it is true.
One thing is absolutely sure. Peter said to Israel on Pentecost, the beginning, according to his 6 Jewish friends in Acts 10 when the gentiles were added through faith -
45 And they of the circumcision which believed were astonished, as many as came with Peter, because that on the Gentiles also was poured out the gift of the Holy Ghost.
46 For they heard them speak with tongues, and magnify God. Then answered Peter,
47 Can any man forbid water, that these should not be baptized, which have received the Holy Ghost as well as we?
and that sure thing was that Peter said the same thing that Mark recorded Jesus saying in their apostolic commission in verses 15-16. So Peter understood the command and that it applied to Judah and Israel whether anyone else understands it or not. Here is proof if you will receive it.
14 ¶ But Peter, standing up with the eleven, lifted up his voice, and said unto them, Ye men of Judaea, and all ye that dwell at Jerusalem, be this known unto you, and hearken to my words: 15 For these are not drunken, as ye suppose, seeing it is but the third hour of the day. 16 But this is that which was spoken by the prophet Joel; 17 And it shall come to pass in the last days, saith God, I will pour out of my Spirit upon all flesh: and your sons and your daughters shall prophesy, and your young men shall see visions, and your old men shall dream dreams: 18 And on my servants and on my handmaidens I will pour out in those days of my Spirit; and they shall prophesy: 19 And I will shew wonders in heaven above, and signs in the earth beneath; blood, and fire, and vapour of smoke: 20 The sun shall be turned into darkness, and the moon into blood, before that great and notable day of the Lord come: 21 And it shall come to pass, that whosoever shall call on the name of the Lord (BEFORE THAT DAY OF THE LORD COME) shall be saved. 22 Ye men of Israel, hear these words; Jesus of Nazareth, a man approved of God among you by miracles and wonders and signs, which God did by him in the midst of you, as ye yourselves also know: 23 Him, being delivered by the determinate counsel and foreknowledge of God, ye have taken, and by wicked hands have crucified and slain: 24 Whom God hath raised up, having loosed the pains of death: because it was not possible that he should be holden of it. 25 For David speaketh concerning him, I foresaw the Lord always before my face, for he is on my right hand, that I should not be moved: 26 Therefore did my heart rejoice, and my tongue was glad; moreover also my flesh shall rest in hope: 27 Because thou wilt not leave my soul in hell, neither wilt thou suffer thine Holy One to see corruption. 28 Thou hast made known to me the ways of life; thou shalt make me full of joy with thy countenance. 29 Men and brethren, let me freely speak unto you of the patriarch David, that he is both dead and buried, and his sepulchre is with us unto this day. 30 Therefore being a prophet, and knowing that God had sworn with an oath to him, that of the fruit of his loins, according to the flesh, he would raise up Christ to sit on his throne; 31 He seeing this before spake of the resurrection of Christ, that his soul was not left in hell, neither his flesh did see corruption. 32 This Jesus hath God raised up, whereof we all are witnesses. 33
Therefore being by the right hand of God exalted, and having received of the Father the promise of the Holy Ghost, he hath shed forth this, which ye now see and hear. 34 For David is not ascended into the heavens: but he saith himself, The LORD said unto my Lord, Sit thou on my right hand, 35 Until I make thy foes thy footstool. 36 Therefore let
all the house of Israel know assuredly, that God hath made that same Jesus, whom ye have crucified, both Lord and Christ.
37 ¶ Now when they heard this, they were pricked in their heart, and said unto Peter and to the rest of the apostles, Men and brethren, what shall we do? 38
Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost. 39 For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call. 40 And with many other words did he testify and exhort, saying, Save yourselves from this untoward generation. 41 Then they that gladly received his word were baptized: and the same day there were added unto them about three thousand souls.
42 ¶ And they continued stedfastly in the apostles’ doctrine and fellowship, and in breaking of bread, and in prayers.
In that transition time for Israel, one could have been a justified believer in Jesus but he would and must be baptized in water to receive the gift of the Holy Ghost and be joined to the church of Jesus Christ, which had it's beginning here.
A gentile believer in Jesus Christ could not have received the Holy Ghost that day, nor any other day before Acts 10 because the door of faith was not opened to gentiles until then. If they could, then Acts 10-11 and many other things in the scriptures makes no sense.
So what these men you guys are so fond of quoting says matters not to me. I believe what God says and I have quoted some of the things he has said in this post. You will be on safe ground to believe it too.