TL;DR: No, it's not a positive act. You'd still be leading people to sin, which is forbidden.
Let's take this up piece by piece. Why this is not allowed is explained mostly at the end.
Islam is considered a pure monotheism according to Judaism.
This is highly debatable. To cite just some examples, during the 11th-12th century CE, the R”i Migash, as quoted by the Meiri, ruled that the Muslims continued in the practices of the pre-Islamic pagans.
Ibn Ezra wrote in his commentary to the Book of Daniel (11:30) that, “The men of Mecca did not turn to his [Muhammad’s] religion until he swore to them that he would not remove the Markolis worship, and it is not necessary to elaborate.”
According to such interpretations, Muslims might practice a covert form of idolatry and their religion isn't pure monotheism. Also there are different sects of Islam, some of which are idolatrous or very close to being idolatrous.
It's famously the only religion a Jew can convert to in the name of their survival because it doesn't require idolatry to do so.
Once again, debatable. Depends on who you ask.
Contrast that to Christianity and you're told it's better to die than be converted.
True, although some claim that Universalism might not be idolatry.
Would encouraging Christians to convert to Islam be considered a positive act because you're encouraging people to transition from an idol based religion to a non-idol based one?
Absolutely not. Explanation at the end.
Judaism doesn't seek out or encourage conversion. This can leave Christians who exit their religion empty of the community aspect of their faith.
Again, debatable. Some Jews in more recent times (and certainly in Ancient times) were much more open to proseltyzing. There is almost never "Judaism does X". But I understand what you're saying. Conversion is long and hard, and Judaism is much more obscure.
Noahidism is lacking. While Noahidism is a movement for non-Jews who acknowledge Torah and Judaism as truth, it isn't a cohesive movement with the same level of community structure. It's a very niche thing not many know about.
I'd strongly contest this. Even still, it would be much better if Noahides were to go out and proseltyze the truth and build their own communities, perhaps with the help of Jews, than do otherwise. Additionally, many Rabbis are opening up to having Noahides present at their Jewish communities, so Noahides can still experience communal events. Lastly, with the trends shifting from physical to digital, many online Noahide communities are popping up and this can also help to obtain a strong community structure. There is more cohesiveness than one may think.
Islam is a pure faith compared to Christianity. Obviously, Judaism is right but Islam is doing things correctly even if it's rooted in incorrect claims. (Rejection of Idolatry, Religious Court System, Charity As a Tool For Sin Forgiveness, Ritual Fasting, Ritual Purity, Clean slaughter of Animals, Reverence for Jewish Prophets, etc.) Compared to Christianity, Islam is arguably as close as a non-Jew can get to Judaism without converting to Judaism.
"Pure faith" is contested and subjective as I said. I'd argue Islam can't do things correctly if its not compatible with the Torah. It has incorrect claims, but more importantly, it contradicts with what B'nei Noach are supposed to do. Citing Chullin 92a, the website Noahide.com claims:
Ulla said: This verse refers to the thirty commandments which the Noachites have accepted. But they keep only three of them. One, they do not draw up marriage contracts for homosexuals. Two, they do not merchandize (human) flesh in the marketplace. Three, they do have respect for the Torah (and for Torah scholars).’
I place emphasis on the bolded part. As Muslims generally claim either that the Torah has been corrupted or things like that, Muslims cannot be said to respect Torah scholars and their abilities to preserve their own texts. Moreover, according to Rabbi Moshe Weiner and colleagues in his nominal book "The Divine Code", gentiles are absolutely not allowed to create new religions, which is what Islam was.
The point is that you would be leading Christians from committing horrifying sins of one kind, so committing sins of other kind. But this would still be leading them to committing sin, and you would be encouraging someone to commit sin, which is forbidden in Judaism as seen in Pirkei Avot 5:18:
And whoever causes the multitudes to sin, they do not give him the ability to repent
All in all, even if Islam on the surface seems close to Judaism, it is still wrong. By doing something like that, you'd be leading people to sin, which is forbidden. As such, one cannot do what you have suggested. It would be much better to educate and teach Noahides about community organizing and the like, than converting them to a false religion. Ultimately, leading people to goodness is what's best for them. Giving them a substitute isn't the solution.