Bible says Mary was highly favored, not full of grace...But even if she was full of grace, what's that mean???
It means Mary was full of graciousness...Joyful, happy...You couldn't make full of grace mean sinless if you were David Copperfield...
Your religion apparently has assigned a different definition to grace that what it actually is...
There's so much to thank God for, here!
Bottom line: this unique neologism Kecharitomene (also called, linguistically, a "nonce word" or "hapax legomenon") is the best Greek word that could have been invented by Divine inspiration to indicate Mary's sinlessness, her being equipped to play her role as the natural source of Christ's human nature, His flesh: human, yet untainted by sin. No other Greek formulation could have conveyed it all.