It occurs in the NT 26 times and is only translated "since" in one verse in the KJV. But I will go with the approximately 300 years of Greek scholarship in the BDAG that ει can mean "since."
I said what I said, deciding who is or is not an obedient believer is God's purview not mine.
In Luke 15:24 it was not the father's purview to determine if his son was spiritually dead or not. The father was using νεκρος/dead figuratively.
As for your repentance discussion I refer you to Heb 6:4-6.
Hebrews 6:4-6
(4) For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,
(5) And have tasted the good word of God, and the powers of the world to come,
(6) If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.