You’re very close. “For some reason” is what people often say when they don’t know why, or when the reason is not important to explain. For example, “For some reason, my phone won’t turn on.” It shows that the cause is unclear or surprising. “For reasons unknown” is a more formal way to say the reason is completely unknown. For example, “The system shut down for reasons unknown.” It is not usually used in casual conversation, but more in formal writing or reports. So both mean the reason is not clear, but “for some reason” is casual and common, while “for reasons unknown” sounds more formal and distant.

"Some reason" implies uncertainty about knowing the reason, but doesn't actually state the speaker's level of knowledge on what the reason may be. The speaker may have no idea, or may have a rough idea while preferring not to go into detail about it. "Reasons unknown" makes it a clear & firm statement specifying that the reasoning is unknown, so if the speaker actually knows the reasoning, then his statement is kind of lying.

The difference is mainly in the connotation- they both mean that the speaker doesn't know why someone did something, but "for some reason" can be neutral to sarcastic. "For reasons unknown" is less neutral and more likely to be sarcastic, or implying that the speaker can't think of any good justification.

Exactly. Some indicates the existence of a justification that, although existent, is not clear. When you use "unknown", it's evident that you have no idea why something occurred.