My post 104, I think, does a good job in covering lots of the other things you have mentioned. But this verse here is not strong at all for proving that we may "fall from grace." The verse literally says that the people who fall from grace are the ones who are "justified by works." Verse 10 speaks of "other-mindedness," and being in confidence that believers will not take another view. IOW, Paul is speaking of doctrine. It is reasonable to conclude that Paul is speaking here only that if a person is truly justified by works, then they must forsake grace in order to do so. This is an argument against a theology of works-righteousness. IOW, "if we are saved by works, we fall from grace." It is not teaching that Christians can "fall from grace." If you want to take it strictly literally, then we must hold that only those who are justified by the law fall from grace.
Thus he says "I sent to know your faith, lest by some means the tempter have tempted you, and our labour be in vain." (1 Thessalonians 3:5)
Indeed, but we should not stretch this into believing that Paul believed his labour to be the deciding factor of salvation. The scripture is clear that Paul plants, others water, but that the "increase" belongs to the LORD. In other words, salvation depends on God's grace alone, and not on the working or desiring of others. We are tools, yes, but we are tools in the hands of God, and He works also inwardly as well as outwardly. Even in the case of repentance, the scripture uses such language as "perchance God will grant him repentance." In John 15, Christ even explicitly says "ye have not chosen me, but I have chosen you," and goes on to declare that He produces fruit in the elect, and preserves it also.
The entire message of this is that salvation is monergistic, even when God threatens and offers and commands or judges. See my post 104.
Which is just what those were in the liberty of Christ, in grace, are clearly warned against doing. Thus they fell from grace by which Christ had made these believers free.
It is not teaching that Christians can "fall from grace."
Thus it only warns believers, as believers, against doing so, and effectually becoming unbelievers.
I think it is clear that believers are warned against making Christ of no effect, or profiting them nothing, and falling from grace by submitting to a gospel of salvation by the merit of moral worthiness as law-keepers, versus faith which is counted for righteousness, and is manifested by working to fulfill the righteousness of the Law.
Indeed, but we should not stretch this into believing that Paul believed his labour to be the deciding factor of salvation....In other words, salvation depends on God's grace alone,
There is no contradiction, and what is being said by Paul is that he feared that his labor as an instrument of grace would be in vain if they gave into the pressure to deny the faith they manifestly had believed.
We have been thru this before, and one can attempt to compel all such texts (such as warns believers as believers against having an evil heart of unbelief, and departing from the living God, making Christ of no effect, becoming of no profit, falling from grace, drawing back into bondage and perdition, versus holding fast the confidence and the rejoicing of the hope firm unto the end, (Heb. 3:6,12,14; 10:38; Gal. 5:1-4) which warn of change of one state into another, forfeiting what faith appropriated), to conform to Calvinistic doctrine, and mean what i see them as most plainly teaching.
And if the Calvinistic doctrine of perseverance of the saints (the P in TULIP) is true, and which is the best case for OSAS and is far better than the antinomianic OSAS too many profess today, then these texts could not mean what i see them as most plainly conveying.
However, despite the temptation, I would rather allow that there is a seeming contradiction or that the Calvinistic conclusions are in error than to compel all such texts to mean contrary to what i honestly see them saying. As said, i see no other alternative unless one dares to hold that such warnings and exhortations are theoretical, as a means of motivating believers to stand firm in the faith. Until one tells them they cannot make Christ of no effect...