I think you know as well if not better than me old friend... :-)
The first thing I would do is to remember that the same person, Jesus Christ, said both things.
Second is that scripture can't be contradictory.
In Mark, Jesus was being chastised by the Pharisee's who had put their own rules and burdens, mostly non-scritpural, on the sabbath.
Their manmade rules and traditions had turned the sabbath from being a delight to making it a burden.
Christ told them the original intent of the sabbath...it was made for mankind to be a benefit for mankind. To be a delight, something healthy and good.
Mar 2:28 “Therefore the Son of Man is also Lord of the Sabbath.”
What's great is that Jesus affirms the sabbath and identfies himself as the Lord of that very same sabbath...or looked at another way as the author of Isaiah 58:13,14!
The original intent. I like that.
Yes....by the time of the Babylonian exile (590 B.C.) and after.....the Rabbinical Councils had begun to take it upon themselves to change quite a few things....all without divine authority. The Pharisees took that ball.......and ran with it and by the first century many things were no longer what the "Original Intent"........ had required. Much of what we hear from Yeshua is constant criticism of the Pharisees and their peculiar take on things.
A cornerstone of the first Covenant from Yahweh was maintaining a tremendous difference between things that were profane (common) and things that were sacred.
Numbers 18:32] And ye shall bear no sin by reason of it, when ye have heaved from it the best of it: neither shall ye pollute the holy things of the children of Israel, lest ye die.
Things that were "Holy"..... and had been so designated by "The Great God of the Universe" were not to be treated as "Common". The Sabbath had been consecrated as something very holy and it contained restrictions on how it should (and should not) be observed. It was to retain its own distinctive nature and not be profaned.
[Exodus 16:23-26] And he said unto them, This is that which the LORD hath said, To morrow is the rest of the holy sabbath unto the LORD: bake that which ye will bake to day, and seethe that ye will seethe; and that which remaineth over lay up for you to be kept until the morning. 24And they laid it up till the morning, as Moses bade: and it did not stink, neither was there any worm therein. 25And Moses said, Eat that to day; for to day is a sabbath unto the LORD: to day ye shall not find it in the field. 26Six days ye shall gather it; but on the seventh day, which is the sabbath, in it there shall be none.
[Isaiah 58:13] If thou turn away thy foot from the sabbath, from doing thy pleasure on my holy day; and call the sabbath a delight, the holy of the LORD, honourable; and shalt honour him, not doing thine own ways, nor finding thine own pleasure, nor speaking thine own words: