why did his transformation drive him insane when no one else's does?
How do we know that no one else's does? Skye/Daisy and Joey were both pretty terrified and freaked out when they turned out to be Inhuman. Maybe they would've had mental breakdowns if there hadn't been anyone to help them understand and cope with their changes.
Remember, until recently, Terrigenesis was something that only happened in controlled conditions, to potential Inhumans who'd been carefully prepared and knew something of what to expect, and who had a support network around them before and after their transformation. Now that it's happening to unprepared people, it stands to reason that those people are going to have difficulty coping with it, and that some will even be driven mad. Those who are open enough with their powers have either been recruited by SHIELD or stuck in the freezer by the ATCU. But Lash/Andrew was able to conceal his change, so there was no one to help him work through it.
Why was he able to delay his full transformation when others are changed instantly?
Maybe because he was triggered by a small, concealed Terrigen crystal used to booby-trap the book, so he didn't get a full dose of the mist? Although the ones getting transformed by the fish oil pills have got to be getting an even more diluted dose, so I don't know if that makes sense.