Second, how do you know that no apostates-to-be ever read this verse and interpret it to mean that they are elect for glory? If apostates-to-be can read this verse and interpret it that way, then just because you interpret it as meaning that you are elect for glory does not show infallibly that you are elect for glory. You could still turn out to be one of those apostates-to-be who mistakenly think the verse is referring to them.
-A8
If the first interpretation is correct then it's rather interesting because this is what the Methodists believe with their two forms of graces. Two graces, two election. Problem solved. Trouble is scripture makes no such distinction. This is manufactured. The burden would be on you to prove to me that there are two forms of election talked about in scripture.
If the second is correct that election is only in reference to elected to grace, then you would have a problem with verses like the following:
or
Isa 65:9 And I will bring forth a seed out of Jacob, and out of Judah one who inherits My mountains; and My elect will inherit it, and My servants will dwell there.
So that leaves you with trying to convince me the scriptures talks about two sets of elections and how you know which election is being talked about.
Under your interpretation, if this was written to those "elected to grace", then Paul would be telling apostates to let the peace of Christ rule in their heart. Obviously this wouldn't make any sense.