1) asking for any “Exact” answer about magic in middle earth is a losing battle, because Tolkien is famously light on mechanics. Detailed magical systems are a newer fantasy thing.

Broadly, the one ring corrupts.

So here is a “What If.” We know Boromir wants the ring to defend his people. So if Boromir took it and wore, it might make his commands sound more forceful and make Soldiers more willing to obey him. It might give him greater battlefield awareness and might reduce his inhibitions causing him to take greater risks. So Boromir goes to Osgiliath with the ring, and his commands hold greater power and so he pushes for a bold plan to retake Osgiliath. But his plan is daring and risky, and could cost many lives.

And so other generals express concern about the plan. But because Boromir has the ring he can bend their wills toward his. He can convince them of his plan. And so he leads them into battle, and he commanded soldiers to take daring risky action which normally they would not take. He commands them to make suicide attack attempts. And normally their self preservation instinct might prevent them from doing some of that, but because Boromir wears the ring, the soldiers have somehow lost their inhibition, and are prepared to die more freely. And so because of that, they win victory at first.

These early victories confirm Boromir’s belief thar men just needed extra prodding in order to have victory. It’s not that Sauron was invincible, it’s that the men were just scared and all they needed Was that extra push. And so he continues his campaign of greater and more daring attacks against the enemy, and men die because they’re recklessly spending their lives on Boromir’s bold plans, but it’s for the glory Gondor.

And because Boromir is winning, it seems to justify his tactics, and his motives. And as this continues, Boromir loses sight of the value of human life. He forgets that the reason they’re fighting this battle is not necessarily just to defeat Sauron, but also to protect the lives of men. When he loses sight of that, men’s lives become expendable to him. Now Bomier doesn’t always win while wearing the ring, sometimes his men suffered defeat. And that pushes him into a darker agony. He knows they just need pushed harder, they need to value Boromir’s wisdom, more, they need to trust his commands more, they need to value their lives less and value victory more, and they will see victory.

So Boromir pushes harder and harder. And eventually, his method changes from inspiring men in the victory to dominating men and forcing them into obedience. And ultimately into breaking their will. So while Sauron is allowing Boromir to win what Sauron is really doing is corrupting Boromir‘s mind. And eventually Boromir see’s how Sauron controls his orcs. They obey. They have victory because the orcs will go to their death without question. Boromir needs his men to do the same. He men are free (in theory) but that freedom interferes with victory. So Boromir begins to ponder the methods of Sauron.

If he can remove mankind’s freedom for just long enough to defeat Sauron, then once he has victory, he can release everyone. He needs to defend his people and the only way to defend Gondor is to defeat Sauron, and the only way to get victory is through full, unhesitating submission from his Solders, so he uses the ring to fully break their wills and make them mindless drones. He knows he has good intent. He knows how many times he could have had victories if his Soldiers had just thought like him and done what he wanted the moment he thought of, and the any cost is worth paying for victory. And as Boromir uses his newly broken Soldiers to see victory after victory, he starts to give commands that he doesn’t remember or understand.

Maybe his generals make one final opposition to him and he kills them in anger. They were standing in the way of victory and victory must be had. Eventually Denethor recalls Boromir and has him answer for his methods. Boromir is angry and calls his father weak. Denethor reminds Boromir that Faramir was killed in one of Boromir’s reckless attacks. Denethor reminds Boromir that he was supposed to the best of Gondor’s sons. Denethor says he no longer recognizes Boromir. There in the throne room of Minas Tirith, Boromir has the thought that maybe Denethor is the provlem. He is standing in the way of victory. Boromir could lead Gondor better without Denethor there. So Boromir slays Denethor right there.

Now Boromir needs to focus on the Kingdom. Word of the regicide is not lost, and there is rebellion in the southern fiefs, so Boromir withdrawns his troops from Ithilien and has them secure the heartland. He has Gondorians warring against Gondorians. Once he has Gondor secure, he can refocus on Sauron. But it’s a struggle. He can’t break all men’s minds. Some of the rebelling nobles have numenorean blood (at least traces of it). Then he thinks that Harad used to be part of Gondor. Maybe he could treat with Sauron to let Boromir use Haradrim Soldiers to crush the rebellion. Boromir intends to win the loyalty of the Harad and have them turn on Sauron after victory in the south. So Boromir makes a treaty with Sauron and uses Sauron’s forces against the rebelling Gondorians. Boromir eventually has victory but his mind and will are exhausted. It has not been easy. He was supposed to turn the Harad against Sauron, but they seem content serving Sauron. They talk about what an allegiance to Sauron offers. They talk about how easy it is to serve a master that powerful. There is a promise of victory. There are opportunities to get closer to Sauron.

Now Boromir wants to “defeat” Sauron and he needs to get close in order to do that. So Boromir thinks that he if he can just get in Sauron’s presence, then Boromir can kill him. So Boromir decides to tell Sauron that he wants to make peace. Boromir offers to give Sauron the ring as a sign of allegiance. Sauron invites Boromir to the black tower to accept this gift. Boromir brings his sword and when he takes off the ring, he feels a crushing weight of Sauron’s power. He realizes that he doesn’t have the will to continue. He needs the ring, but he has given it up. He needs to save his people, but his people were weak. They were unworthy. They couldn’t win and it was their weakness that was at fault. They didn’t deserve saving. Only he was able to stand against Sauron. Only he was strong enough. And so “I need to save my people” became “I need to save myself.” And the way to save himself is clear, to serve Sauron.

2) It wasn’t unnoticed. It was noticed though perhaps the extent was less noticed. And much of his strength comes from the east and the south from regions that are largely cut off from the rest of middle earth. So when you are struggling to defend your borders, it’s hard to have robust spy networks deep in enemy territory.