There is no reason to add this "messenger" to Paul's being, as it seem your view does. Paul stands before God alone. The mesenger of Satan was to keep Paul humble, as he clearly indicates. Nothing more.
You are welcome to only observe Paul. Paul did not make that case for himself.
So, somehow, the messenger makes Paul "whole"??? How does that work?
I would suggest that this "dilemma" is the dilemma of every sinner. The presence of "lawlessness" in the form of "evil present" in the "internals" of us all is what makes all of us "lawless." This is why no person can be 'justified' under the law. And Grace makes no provisions or excuses for sins either.
The basis of the complaint that the anti eternal security crowd makes is this: That there is then no "judgment" for believers who sin, and sin therefore is irrelevant. This is in fact
a very valid complaint. Their own folly however is that they believe that they are not sinners.
The basis of judgment for sins remains upon every believer. Grace has not justified a single sin amongst the lot of us, NOR does Grace just 'gloss over" sin for us. This is a common sight from the eternal security camp, that being "in Christ" means that God doesn't see "my" sins, but DOES see everyone else's that doesn't believe. And this makes God into just another common hypocrite, not seeing "our sin" but seeing "everyone else's sin." That's NOT how it works and God is not a hypocrite. God is no respecter of persons when it comes to sin and judges us all "as sinners." This part of the christian equation doesn't change after salvation. And in fact if we observe Paul, his own condition may have gotten WORSE after salvation, as he saw himself the "chief of sinners" AFTER salvation.
The only credible angle that we can view that with is as I noted earlier, that an Apostle is "singled out" for even greater "internal attacking" by the tempter, placing same "within" the Apostle to do so.
We can even read in Revelation this:
Revelation 2:2
I know thy works, and thy labour, and thy patience, and how thou canst not bear them which are evil:
and thou hast tried them which say they are apostles, and are not, and hast found them liars:
I'd say any claimed "Apostle" will in fact confess that temptation of the tempter is within them, because that is a fact, and without that "trial" and such speaking that it is a fact for them personally, they can NOT be an Apostle.
It is "essential" that an Apostle "understand" his own internal dilemma. The "presence" of A LIAR is in fact within them to a greater degree than anyone else. And if such a called out one can not own up to this fact, they are not AN APOSTLE. If they can not step into Paul's own shoes, they are LIARS if they can not speak this truth.
Paul OWNED UP. He 'fessed up' with the fruit of simple HONESTY. And in that process he 'exposed' his internal dilemma for us all to observe, and to consider for ourselves.
What about what Jesus said of those who believe: they HAVE eternal life (Jn 5:24). Did Jesus include a disclaimer about any believer who had a "messenger of Satan"? No, of course not.
We do 'have,' present tense, eternal life. But that "life" is not extended to the tempter, who's workings are "internal" within us all. This is very simple to observe.
It is fallacious to even consider this messenger of Satan as being "part of the man" of Paul, or anyone else. The messenger of Satan did not become a part of Paul, as is being assumed.
Temptation was in the flesh of Paul. Evil present was "within" Paul. Paul had a messenger of Satan in his own flesh. It is impossible NOT to see it. And if we do see this AND consider this to be also a fact for ourselves, then it is we who are blinded to the fact by our own "internal enemy." Only the fruit of "honesty" can speak of this matter. I honestly say that I am tempted by the tempter, internally. Therefore where is the tempter, but internal? This makes "me" a child of God who "bears" the tempter in my own flesh. So, no, it's not just me. Nor was it "just" Paul.
I would utterly refuse to "justify" the tempter in anyone. Even less, to claim that working "legal, obedient OR under Grace."
The sheet of Grace does not extend that far and never will.
Did Paul say or insinuate that he was being indwelt by this "messenger of Satan"? No, he did not.
I would suggest that is exactly what Paul said in 2 Cor. 12;7.
It should be pretty obvious that Paul was speaking of one of Satan's fallen angels assigned to torment Paul. Not to indwell him. There are no verses that tell us or suggest that a believer can be indwelt by a demon.
It does not take some spiritual giant to understand that "temptation" transpires within and that temptation is of and from the tempter.
It only takes honesty to realize that places that operation and operator within to do so.
But you might consider that that very working will buck and fight that disclosure in anyone, and attempt to blind them to it.
The reason should be obvious; we are indwelt by the Holy Spirit. How in the world could a demon co-habit with the Holy Spirit? That is nonsense.
Believers are still sinners after salvation and sin is still of the devil no matter how you care to slice it. This notion that anyone is "entirely holy" when the Apostle of our Lord exposed that he had "evil present" with him is utter nonsense.
Evil present within no person will ever be "justified" "legal" or "obedient" or even less, under Grace.
And Paul assuredly had "evil present" with him by his own very public admissions.
Romans 7:
21 I find then a law, that, when I would do good,
evil is present with me.
No one "escapes" this law after salvation.
You may think it so, but the blanket of Grace and Salvation does NOT extend to the tempter. Never has, never will, never happened.
No one can justify the "entirety" of themselves while in the flesh by any measures.