I am no trained NT Greek grammerian. The technicalities you highlight, I will not comment on here.
The issue is whether the warning of Matthew 5:23,24 (for an example) is to be altogether ignored by the church because it is intended only for OT Israelites under the Law.
We should know that Christians for whom eternal redemption has been solved in the affirmative, will suffer loss or be rewarded after the second coming.
If anyone’s work which he has built upon the foundation remains, he will receive a reward;
If anyone’s work is consumed, he will suffer loss, but he himself will be saved, yet so as through fire. (1 Cor. 3:14,15)
If the SAVED church person suffers loss, it must be a TEMPORARY suffering of loss.
If it is temporary then it must have a point of termination, concluding.
" he will suffer loss, but he himself will be saved,".
There is no question that THIS is written to "To the church of God which is in Corinth, to those who have been sanctified in Christ Jesus, the called saints, with all those who call upon the name of our Lord Jesus Christ in every place, who is theirs and ours:" (1 Cor. 1:2).
That is enough to warn us churching saints to think "I must consider that though I have the gift of eternal redemption that does not tie God's hands from teaching me a disiplinary lesson in the age to come." The First Corinthian letter would only warn of something SIMILAR to what Matthew 5:23,24 already warned, regardless of for whom Matt. 5 was intended.
Ie. "If I argue that the warning of Matthew 5:23,24 is only for Law keeping Old Testament Israel, what good does it do me? Virtually the same kind of warning is clearly written to those under NT grace in the church. I gain exactly nothing to believe Christ wasn't talking to me there in the Sermon on the Mount."
Other portions in Luke indicate similarly. After the second coming of Christ a Christian servant might suffer punishment according to whether he knew better or didn't. If he did clearly know better, his disipline will be stronger. If he did not know better, his discipline will still be, but be lighter.
Luke 12:43-48 speaks to Christ second coming and reckoning with the behavior of waiting servants.
Blessed is that slave whom his master, when he comes, will find so doing.
Truly I tell you that he will set him over all his possessions.
But if that slave says in his heart, My master is delaying his coming, and begins to beat the male servants and the female servants and to eat and to drink and become drunk, the master of that slave will come on a day when he does not expect him and at an hour which he does not know, and will cut him asunder, and will appoint his portion with the unbelievers.
And that slave who knew his master’s will and did not prepare or do according to his will, will receive many lashes; But he who did not know, yet did things worthy of stripes, will receive few lashes. But to everyone to whom much has been given, much will be required from him; and to whom much has been committed, they will ask of him all the more.
1.) The faithful well serving Lord's servant will receive a reward of being over all the Lord's possessions - the millennial reign.
2.) The unfaithfully serving Lord's servant who acted like the worldly unbeliever will be punished.
3.) "Many lashes" is most likely not infinite lashing. Neither is "few lashes" infinite lashing. This indicates whether harsh lashing or less harsh lashing, both are terminal. Both disciplines must come to a terminal conclusion.
4.) The severity of discipline is based on the light, awareness, and knowledge the servant had of the will of the Lord. "How much did he know better ?"
5.) HOW terminal is left an unknown. There is room for understanding any length of time for punishment. It should not be more than the length of time before the new heaven and new earth in which ONLY righteousness dwells. (2 Pet. 3:13)
We should know that the time after His second coming to the new heaven and new earth is said to be a thousand years. (Revelation 20:1-6)
Possible Pushback: "Well, well, Luke 12:43-48 is only written there for Law Keeping Old Testament Jews as well." That would be unreasonable to believe imo. Peter asks about His teaching on watchfulness and faithfulness. And the Lord's response is to all who expect His coming. How can that NOT mean churching believers under grace?
"But know this, that if the master of the house had known in what hour the thief was coming, he would not have allowed his house to be broken into.
You also, be ready, because at an hour when you do not expect it, the Son of Man is coming.
And Peter said, Lord, are You saying this parable to us, or also to all?
And the Lord said, Who then is the faithful and prudent steward, whom the master will set over his service to give them their portion of food at the proper time?
Blessed is that slave whom his master, when he comes, will find so doing.
Truly I tell you that he will set him over all his possessions.
But if that slave says in his heart, My master is delaying his coming, . . . etc. etc. etc. "