He does lie all the time, though. Even if some exposition comes from him.
I don't believe the others are there to protect the source. They are there for Jacob to prove the MiB wrong. However, I believe they do know on some level that Jacob allows people to be brought to the Island, and that's why they won't allow people to leave. That's why it would make Ben look weak as a leader. Richard has told Ben that Jacob brought people there. Because any and all of the newly arrived people could be the real candidate.
My theory about the Cabin (kinda explains the discrepancies, I think). It used to be Richard's and Jacob's meeting place. (Ilana knew it to be Jacob's home, that's why she went there.) That's why it had a ring of ash around it, so that Richard's and Jacob's activities/meetings are kept from the MiB, not to keep Jacob safe. (He didn't need to be kept safe.) Later, Jacob abandoned it. Who broke the ash circle isn't really important. It could have been anyone. Ben knew where the cabin was because Ben Linus knows things. Maybe he followed Richard, what do I care. However, the look of Richard's face is telling. He's very surprised Ben and John were going to see Jacob, but he probably thought the circle was still intact. This is some of the last we see of Richard for a long time. Ben and John are duped, twice, by the MiB. I don't think Jacob even knows. The thing is, even if Richard told him, I don't think it would have mattered. I believe the season 5 finale made it pretty obvious he was tired of the job and wanted to move on. Jacob is all about choice, because he is bored and wants people to live out their lives without his interference.
Now if that seems like weak character motivation, keep in mind that this is a guy who for thousands of years brought people to the Island letting them be killed by his dead brother - without even interfering. He probably thinks he is too hands-on just having Richard as a representative.
Btw, I believe the cabin moves because you have to know/feel where it is, or be in the right frame of mind, to find it. Same with the Lighthouse and The Heart of the Island. MiB is a smart guy who knew the Island well, and yet he couldn't find the Heart even though he looked for ages.
The eye could still be Locke. On Twin Peaks, creator/head writer Mark Frost himself played the person who shot Dale Cooper, but needless to say they found another shooter for the actual plot. Or it could be someone else. This is the episode where Hurley starts to see ghosts for real, so it could just be a random ghost. I can live with it.
EDIT: Oh, and I believe Horace had nothing to do with building the cabin. It was just an old cabin. Or it was built by Jacob, doesn't matter. My theory: MiB impersonated Horace because he knew John was extremely curious about all things DHARMA. He also had to guide John to the cabin again so that he could impersonate Jacob inside. For that reason, after putting the map/sketch in Horace's pocket, he let John know that Horace built the cabin.