Do the people God sends to challenge His people (for His Glory) have salvation?

I think the rest of the letter to the Romans can give a thorough answer to this question. Let's bear in mind, first of all, that the book is written by Paul (who formerly persecuted Christians, but was called by God to become his apostle to the Gentiles) to the church in Rome, the capital of the Empire that was occupying the Holy Land, persecuted Jews and Christians, and was the symbolic successor of all the big bad empires of the Old Testament. It's safe to say that the people who first read the letter would be very interested in knowing whether they, as Romans, could be saved, and what might happen to the other Romans who counted themselves against Christianity.

The theme of the book is set out in the first lines:

I have an obligation to Greeks as well as barbarians, to the educated as well as the ignorant, and hence the eagerness on my part to preach the gospel to you in Rome too. For I see no reason to be ashamed of the gospel; it is God's power for the salvation of everyone who has faith - Jews first, but Greeks as well - for in it is revealed the saving justice of God: a justice based on faith and addressed to faith. As it says in scripture: Anyone who is upright through faith will live. (Romans 1:14-17, NJB)

So the text affirms, strongly, that salvation is available to anyone; and Paul's personal history proves that even previously being an "enemy" does not mean that one is cursed forever. Paul goes on to say that "there is no favoritism with God" (2:11), quoting Deuteronomy 10:17 (which goes on in v19 to say "Love the stranger then, for you were once strangers in Egypt"). Even worse,

Are we any better off [than the condemned]? Not at all: we have already indicted Jews and Greeks as being all alike under the dominion of sin. (3:9-10)

However, faith is available to everybody - it is not the case that acting as an enemy of God cuts one off from salvation. If this were true then there would be no hope for anybody, since we are all slaves of sin. This theme is expounded throughout the letter. In 9:25-26, Paul loosely quotes Hosea, saying

I shall tell those who were not my people, "You are my people," and I shall take pity on those on whom I had no pity. And in the very place where they were told, "You are not my people," they will be told that they are "children of the living God."

Even the governing authorities of Rome are meant to be respected (13:1-7, and likewise 2 Timothy 2:1-2, Titus 3:1). So it is clear from these passages that even those who set themselves up against God may be saved, in the same way as anybody else. This doesn't mean that they necessarily do enjoy salvation, just that they are not "cursed". Paul says in 13:14, "Bless your persecutors; never curse them, bless them," which is of course a direct echo of the words of Christ, in Matthew 5:43-44, "You have heard how it was said, You will love your neighbor and hate your enemy. But I say this to you, love your enemies and pray for those who persecute you."

The remaining issue from your question is whether people like Pharaoh might be "automatically" saved or blessed because they are carrying out God's will. Paul dismisses this possibility in Romans 3:7-8,

You might as well say that if my untruthfulness makes God demonstrate his truthfulness, to his greater glory, then I should not be judged to be a sinner at all. In this case, the slanderous report some people are spreading would be true, that we teach that one should do evil that good may come of it. In fact such people are justly condemned.

So, no such luck for Pharaoh: just because God managed to make something good out of the bad things he did, doesn't mean he gets a free pass. But, much more importantly, the "enemies" of your question have just as much access to salvation as anybody else.