1) Something that is sacred is not just where God touches the material world, but is something SET ASIDE for God. That is what "sacred" means -- "to set aside". Us touching a sacred object does not make us sacred. Sacredness is intrinsic to the person or object itself as it or he is set aside for God special purposes.
2) On Mary's Assumption, the doctrine is derived from the understanding of the Sacred as mentioned in the previous post linked above. In addition, as you mentioned, not everything is in the Bible, thus the question, "where's that in the bible" is a false question. But, all the Marian Doctrines are derived from the Bible.
3) As to why Jesus couldn't have healed Mary the moment he was conceived and cleansed her of Original Sin? Well the point is that the Ark of the Covenant cannot be contaminated in the First Place. One cannot take a contaminated vessel and cleanse it for God to bring His special presence. It must be pure and unblemished to begin with. The sacrifices of the lamb in the Old Testament had to be pure and unblemished to begin with, not cleaned up later.
And, I would ask this question of her, "why couldn't Jesus have preserved Mary from original sin at the moment of her conception." And that is the point. Mary was not "cleansed" of original sin, but PRESERVED from the stain of original sin.
4) AS for Jesus living in a world of darkness, so did Mary. But there was NO darkness IN Jesus; and there was no darkness IN Mary, nor could their be if she was to be the Ark of the Covenant.
BTW, light and darkness do co-exist. God is everywhere. There is no place God is not at, even hell. God is in us even though we are not perfect. Mary, however, had to be perfect and sinless or she could not have been the Ark of the Covenant.
5) On her claimed that Mary didn't have to be free of sin because God was inside her misses the point. God is in us, but we are not sacred (set apart) to be the Ark of the Covenant.
Mary has a particular and singularly unique role to play. We do not play that role. For God Himself to reside literally in Mary's womb is NOT the same thing as the Holy Spirit indwelling in us. As the Old Testament teaches us, this particular point deals with a total lack of blemish, a total purity. If Mary had sin, then she could not have been the Ark... JUST AS if Jesus had sin he could not have been the Lamb of God.
The Ark and the Lamb both had to be totally pure and unblemished. There is NO question of this teaching in the Old Testament and no credible theologian of any stripe would disagree with this.
The basic problem is that so many people are ignorant of the Old Testament. One cannot understand much of anything without understanding the Old Testament, and one cannot understand the Old Testament except in light of the New.
I hope this helps, but keep in mind what Jesus said: "there are none so blind and those who will not see."
CONCLUSION
What Christian would not give the grace of sinlessness to his/her mother if he/she had the power to do so?
What Christian would not bring his/her mother into Heaven, body and soul, if he/she had the power to do so?
These Marian doctrines are NOT "non-Scriptural" as non-Catholic Christians seem to believe. Jesus Christ was only fulfilling the Fourth Commandment (Exodus 20:12) to "honor" His mother by giving her these unique graces and privileges. To say Christ did not give these graces to His mother when it was possible for Him to do so, is to accuse Him of a sin of omission against the Fourth Commandment. Therefore, since Christ did not sin (Hebrews 4:15), Mary has to be sinless and was assumed into Heaven for God Himself would not disobey one of His own commandments.
God Bless