You have not even dealt with text of Rom 8:13. That alone says much about the weakness of your position since you refuse to deal with it. Shall I await for your attempt to explain away the meaning of this verse or will you continue to conveniently ignore for the sake of clinging to your doctrine?
How can if mean since? If - means possibility. Since - means certainty. Two totally different meanings. You twist the English language. Like I wrote earlier, this verse is a 1st class conditional sentence which indicates cause and effect. If this condition - than that consequence. Your explanation holds no water whatsoever.
Unless Rom 8:13 exists without context, the context is relevant (or maybe I should say IF it exists within a context...(since it does)). Your claim it stands alone and therefore must be dealt with alone is simply wrong. And like I just demonstrated within the parenthesis, IF can also serve in place of SINCE at times, and often does, in the English language as well as in perhaps most other languages.
An IF statement is propositional rhetoric. So is a SINCE statement. You are correct that the one assumes a certainty, while the other only assumes a given. Nevertheless, both propose that the result is dependent on the assumed. Consider these two statements: "If I am right about that, your claim is superfluous." "Since I am right about that, your claim is superfluous." Both those statements may be true, and neither keeps the other from being true.
As to your claim that I haven't dealt directly with Rom. 8:13 that is what I have been doing all along. But to repeat myself (in different words), Rom 8:13: "For if you live according to the flesh, you will die; but if by the Spirit you put to death the misdeeds of the body, you will live." uses IF in two propositional statements: Assuming you live according to the flesh, you will die. Assuming you put to death the misdeeds of the body, you will live. The truth of the second part of each phrase logically depends upon the truth of the first.
The persons addressed in the verse (see "context") are the members of the Roman church. This will include not only the actually regenerate, but those who claim to be. IF those who claim to be regenerate live according to the flesh, they will die.
Likewise, {IF those actually regenerate live according to the flesh, they will die} is a possibility if it is referring to a physical death, as opposed to spiritual. This is not my interpretation, but I reference it as a possibility.
So far, I see no way that Rom 8:13 proves me wrong about God accomplishing whatever he sets out to do, nor about the ability of anyone to overcome God's purposes, nor about any of those the God has chosen for eternal life being lost.
Perhaps it would be worth mentioning at this point that it makes no sense to call a supernatural being "God", as in THE self-existent omnipotent CREATOR, if anyone is able to undo what he has set out to do.