Look, Islam has been around since Adam, and it didn't just pop into existence when Muhammad showed up. You really ought to know that if you've spent even a tiny amount of time studying Islamic faith or any religion for that matter.
Exactly. This idea that Islam has existed since Adam is a belief, not an established fact.
It's just one group's conviction, and nobody else is obligated to buy into it.
All the prophets were Muslims. After Moses passed away, a sect formed that we call Jews today, and after Jesus was ascended, a Jewish splinter group called Christians emerged. The scholars alongside those prophets were believers who held the same faith as Muslims do now, though their specific laws—their Sharia—were different from what we have today.
Every prophet (if we're using that term) was a monotheist, not a "Muslim" in the way the word is used today.
Being a monotheist and being a Muslim aren't the same thing.
Calling Judaism and Christianity "sects" is honestly just ridiculous.
Also, you should realize that neither Judaism nor Christianity existed before Moses and Jesus. People like Jacob, Abraham, Ishmael, Isaac, Jonah, Noah, Lot, Solomon, and the rest—they all came before Moses and Jesus. May peace and salvation be upon them.
Solomon before Moses???
Islam didn't exist before Muhammad either. Everyone listed who lived before Moses and Jesus could maybe be called monotheists, but they definitely weren't Muslims.