I'm going to jump in on a couple of things. The first is Mary's being full of grace.
For starters, it would seem the Catholic Church 'invented' that Mary was 'full of grace'. In particular, except for the DRB, eg, Douay-Rheims (I think the Catholic Church stopped using that translation not long after Vatican II - I'm probably wrong as I'm was never a Catholic) Luke 1:28 has no mention of 'grace' whatsoever! I checked multiple bible translations that I have downloaded on e-Sword - KJV, ESV, NLT, NASB, NIV, ISV, NRSV-CE and DRB. Compare the differences - (by the way, e-Sword is free for downloading, most Bible translations are free)
28 And the angel came in unto her, and said, Hail, thou that art highly favoured, the Lord is with thee: blessed art thou among women. (KJV)
28 And he came to her and said, "Greetings, O favored one, the Lord is with you!" (ESV)
28 Gabriel appeared to her and said, “Greetings, favored woman! The Lord is with you!” (NLT)
28 And coming in, he said to her, "Greetings, favored one! The Lord is with you." (NASB)
28 The angel went to her and said, "Greetings, you who are highly favored! The Lord is with you." (NIV)
28 The angel came to her and said, "Greetings, you who are highly favored! The Lord is with you!" (ISV)
28 And he came to her and said, 'Greetings, favoured one! The Lord is with you.' (NRSV-CE - Catholic Edition)
28 And the angel being come in, said unto her: Hail, full of grace, the Lord is with thee: blessed art thou among women. (DRB)
The Douay-Rheims uses 'grace' a 2nd time in verse 30.
30 And the angel said to her: Fear not, Mary, for thou hast found grace with God. (DRB)
In doing a quick search of the 4 Gospel and Acts in e-Sword, most versions do not use 'grace' except in connection with Jesus and God. In particular, many versions 1st use of 'grace' in the Gospels is Luke 2:40 describing Jesus' childhood -
Luk 2:40 And the child grew, and waxed strong in spirit, filled with wisdom: and the grace of God was upon him. (KJV)
Other versions 1st use of 'grace' in the Gospels is in John 1:14 -
Joh 1:14 And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth. (KJV)
It is JESUS that is full of grace! Draw your own conclusions about Mary's being filled with grace.
My second concern is tying ('manufactured') grace in Luke 1:28 to grace in Hebrews 4:16. The issue is taking a verse out of context. Read the preceding verses of Hebrews to get the context of verse 16 -
Heb 4:14 Seeing then that we have a great high priest, that is passed into the heavens, Jesus the Son of God, let us hold fast our profession.
Heb 4:15 For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin.
Heb 4:16 Let us therefore come boldly unto the throne of grace, that we may obtain mercy, and find grace to help in time of need. (KJV)
It's clear that the grace in verse 16 speaks of Jesus IS the throne of grace...no reference whatsoever to Mary.
Remember, too, that Hebrews was written after Jesus ascended into Heaven to show Israel that Jesus WAS the Messiah they've been looking for and that FAITH in Him is the way to salvation. In the Gospels, Jesus was all about showing Israel directly that HE was the promised Messiah as he fulfilled all the prophecies about the Messiah in the Old Testament. The leaders simply couldn't believe that the lowly carpenters' son WAS the promised Messiah!