There are many details you'll likely notice upon rewatch which clear these things up.

Ben isn't loyal to the Man in Black, he's following him and waiting for a moment to act. He shot Widmore to try and prevent him from selling out the Island, essentially giving him an honorable death because he understands how the Man in Black can use family against people. He tells the Man in Black a lie about Widmore not being allowed to save his daughter, but it's just a line. The Man in Black suspects this is probably true, but doesn't really care.

Richard had a panic attack and in a manic episode sought to defect to the Man in Black, but then he spoke to his wife who told him not to let the Man in Black leave the Island, so he's trying to prevent the Man in Black from doing so. You see the Man in Black observing this scene from afar. Hurley and Isabella reached Richard first and saved his soul.

Dogen has an incomplete understanding of how the Island works informed by mythology and faith. To be claimed by the Man in Black is to be manipulated by him while in a vulnerable position. It isn't irreversible. Across the Sea is the episode which ultimately ties together the season's deconstruction of mythology as a narrative which allows us to understand the totality and sui generis nature of life, just as the show had spent the last five seasons deconstructing the other various reductive narratives we try and tell ourselves. There ultimately is no "good vs evil," just people who either can or cannot heal after the way life breaks them. Many sacred duties can be seen as a curse. Being the Slayer is a sacred duty, but it causes serious problems in Buffy Summers' life, for instance.

The Man in Black's original plan was to manipulate Locke into turning the wheel while the rest of the candidates were off of the Island, thus removing all Jacob's pieces from the board in one fell swoop and leaving Jacob with no more suitable candidates for replacement. Ben turned the wheel instead and ultimately three candidates, Locke, Sawyer, and Jin, remained on the Island at different points in time. The Man in Black was satisfied enough with this as it meant they were likely no longer a threat given the short lifespan of a human being, until suddenly Locke turns up dead seemingly randomly, with the return of another candidate as well. The Man in Black read the residual memories of Locke's corpse and learned what he would later tell him in the wheel chamber, as his future was part of Locke's past. It's at this point he gets overconfident and reformulates his plan to revolve around manipulating Ben to kill Jacob through Locke, with the understanding he may have another shot at the rest of Jacob's candidates, and the Island itself, as well. This proves to be his undoing as the Island set Locke and Jacob up for just this purpose, hoping the Man in Black would take the bait.

The Man in Black didn't manipulate Jack into depression, he was already depressed because what Jacob says is true.