The "Mosaic Law" - the law that was given to Moses directly from God's mouth - applied to human beings. When Christ became human he was "born under the law" and therefore subject to it. Currently I do not believe he is any longer subject to it. But human beings should obey it if they desire to live in a holy fashion. Saying that the law has been set aside in this case is like saying the law of ritual purification regarding child birth has been set aside for a man. The law never applied to the man - only to women, so of course it's set aside in his case. However, that does not abolish the law since it still carries its proper application.
Heb 7:18-19 needs to be read in context...
Heb 7:11 If therefore perfection were by the
Levitical priesthood, (for under it the people received the law,) what further need was there that another priest should rise after the order of Melchisedec, and not be called after the order of Aaron?
Heb 7:12
For the priesthood being changed, there is made of necessity a change also of the law.
What Law was changed?
Num 18:1 And the LORD said unto Aaron, Thou and thy sons and thy father's house with thee shall bear the iniquity of the sanctuary: and thou and thy sons with thee shall bear the iniquity of your priesthood.
Num 18:2 And thy brethren also of the tribe of Levi, the tribe of thy father, bring thou with thee, that they may be joined unto thee, and minister unto thee: but thou and thy sons with thee shall minister before the tabernacle of witness.
Num 18:3 And they shall keep thy charge, and the charge of all the tabernacle: only they shall not come nigh the vessels of the sanctuary and the altar, that neither they, nor ye also, die.
Num 18:4 And they shall be joined unto thee, and keep the charge of the tabernacle of the congregation, for all the service of the tabernacle: and a stranger shall not come nigh unto you.
Num 18:5 And ye shall keep the charge of the sanctuary, and the charge of the altar: that there be no wrath any more upon the children of Israel.
Num 18:6 And I,
behold, I have taken your brethren the Levites from among the children of Israel: to you they are given as a gift for the LORD, to do the service of the tabernacle of the congregation.
The Law that says only a direct descendant of Aaron of the tribe of Levi can be a Priest, specifically the High Priest. This is teh reason for the very next verses...
Heb 7:13
For he of whom these things are spoken pertaineth to another tribe, of which no man gave attendance at the altar.
Heb 7:14
For it is evident that our Lord sprang out of Juda; of which tribe Moses spake nothing concerning priesthood.
We are not talking about the whole of the Law here, we are talking about the Law that says that a Priest must have the DNA of Aaron and Levi. Christ did not have that DNA, He was from the tribe of Judah. So without the changing of the Law, He could not be High Priest. Christ will NOT VIOLATE His own Laws.
Heb 7:15 And it is yet far more evident: for that after the similitude of Melchisedec there ariseth another priest,
Heb 7:16 Who is made, not after the law of a carnal commandment, but after the power of an endless life.
Heb 7:17 For he testifieth, Thou art a priest for ever after the order of Melchisedec.
Heb 7:18 For there is verily a disannulling of the commandment going before for the weakness and unprofitableness thereof.
Heb 7:19 For the law made nothing perfect, but the bringing in of a better hope did; by the which we draw nigh unto God.
Now instead of lifting these two verses out of context, let's read on...
Heb 7:20 And inasmuch as not without an oath he was made priest:
Notice that "inasmuch" is referring to Him being made Priest. It is not referring to the Ten Commandments or the whole of the Law.
Heb 7:21 (For those priests were made without an oath; but this with an oath by him that said unto him, The Lord sware and will not repent, Thou art a priest for ever after the order of Melchisedec
Heb 7:22 By so much was Jesus made a surety of a better testament.
Heb 7:23 And they truly were many priests, because they were not suffered to continue by reason of death:
Heb 7:24 But this man, because he continueth ever,
hath an unchangeable priesthood.
The subject is the Priesthood and the fact that the Law had to be changed to legitimize one from teh tribe of Judah being High Priest. Christ was the LORD who spoke those Laws to Moses and WILL NOT violate His own Law, therefore the Law had to be changed.