So why did Jesus say that he wasn't sinless (Mark 10:18)?
Already asked and answered; you didn't understand.
And why didn't he stone the adulteress if he was sinless (John 8:7) given that he said that every jot and tittle of OT law still applies till heaven and earth pass when all is fulfilled (Matt 5:17-19), including the commandment to stone adulteresses (Lev 20:10) and to terminate the pregnancies of adulteresses (Numbers 5:20-28)?
Which law do you think Jesus means?
The one given by those of old,* or the law given by Jesus?
Matthew 5:18 For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled.
21 Ye have heard that it was said by
them of old time, Thou shalt not kill; and whosoever shall kill shall be in danger of the judgment:
22
But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca, shall be in danger of the council: but whosoever shall say, Thou fool, shall be in danger of hell fire.
5:27 Ye have heard that it was said
by them of old time, Thou shalt not commit adultery:
28
But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.
5:31 It
hath been said, Whosoever shall put away his wife, let him give her a writing of divorcement:
*
32
But I say unto you, That whosoever shall put away his wife, saving for the cause of fornication, causeth her to commit adultery: and whosoever shall marry her that is divorced committeth adultery.
5:33 Again, ye have heard that it hath been said
by them of old time, Thou shalt not forswear thyself, but shalt perform unto the Lord thine oaths:
34
But I say unto you, Swear not at all... 37 But let your communication be, Yea, yea; Nay, nay: for whatsoever is more than these cometh of evil.
5:38 Ye have heard that
it hath been said, An eye for an eye, and a tooth for a tooth:
39
But I say unto you, That ye resist not evil: but whosoever shall smite thee on thy right cheek, turn to him the other also.
5:43 Ye have heard that
it hath been said, Thou shalt love thy neighbour, and hate thine enemy.
44
But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you
____________
* Matthew 19:7 They say unto Him, Why did Moses then command to give a writing of divorcement, and to put her away? 8 He saith unto them,
Moses because of the hardness of your hearts suffered you to put away your wives:
but from the beginning it was not so.
The Kingdom of God is this higher standard. From the beginning.