...Why old Christians have to add 18 statements to Mark to finish the story?
All your questions, aren't really questions.
They (Christians) didn't "add", nor "take away", as it is forbidden. They faithfully recorded by inspiration of God.
First of all, Psalms 12:6-7, etc already showed that JEHOVAH Elohiym preserves JEHOVAH Elohiym's words.
Manuscripts (MSS, abbreviated) evidence for the reading (or Gospel ending) of Mark 16:9-20 KJB-
3 of the so-called Great Uncials : A (Alexandrinus), C (Codex Ephraemi Rescriptus), D (Codex Bezae Cantabrigensis), and many other Uncials: E, F, G, (H), K, (L), M, S, U, V, (W), X, Y.
It is in - Gamma, Delta, Theta, Pi, Sigma, (Psi), Omega, (099), (0112).
In thee vast # (1,000's of MSS) of "Cursives", being in "MAJORITY", fam 1,13.
Old Latin - aur, c, d-suppl, ff2, g1,2, l, n, o, p, r2, & Vulgate (1,000's of MSS)
SYRIAC - peshi.tta, curetonian, harclean, palestinian.
COPTIC - sahidic, bohairic, fayyumic.
Gothic, Armenian, Ethiopic.
It is extant (available) in 047, 055, 0211, 0233?, 0257.
Early bishops (so-called ECF), who claimed direct succession of the Apostles never left it out-
TATIAN, Diatessaron, (I 10:125,126,128,129), in entirety.
IRENAEUS, Against Heresies (I 1:426), vs. 16:18
TERTULLIAN, Resurrection of Flesh (I 3:584), vs. 16:19; Against Praxeas (I 3:627), vs. 16:19
CYPRIAN, 7th Council (I 5:569), refers to vs. 16:17,18; Gospel of Nicodemus (I 8:422,432,436,445), vs. 16:15-19
APHRAHAT, Demonstrations (III 13:351), vs. 16:16-19
APOSTOLIC CONST. (I 7:457,479), vs 16:16-19
AMBROSE, The Holy Spirit (III 10:133,134), vs. 16:15-18; Const. of Faith (III 10:216), vs. 16:15; Repentance (III 10:335), vs. 16:17,18.
Nothing in indexes of the ANPF of pre-400 AD father quoting up to vs. 8 and stopping.
Main line Bibles always had it: AV (KJB), HF & CR*; Tyndale, Great, Geneva, Bishops, Stephanus, Th. Beza, Elzivir bro., E, 238, 239, 440, 475-2nd. cor., 477, 486, 488, 489, 1071, & etc.
Basically in 10,000's of MSS, only 'missing' in: Aleph (Sinaiticus) [which omits quite a bit], B (Vaticanus), the 2 most 'emended' (corrupted) Codices and in 'pc' (UBS cites 1 lone cursive*), (k), sin, sa-ms, Arm-mss.
As for Vaticanus (codex B), there is the exact 'space' for it in the MSS: -
Should the Longer Ending of Mark (Mark 16:9-20) be in the Bible? - King James Version Today
Between Luke and John, there is no such 'space':
For more detail, please see brother Will Kinney's page -
https://brandplucked.webs.com/mark16920.htm