You have the right idea. In American English, normally this phrase is used while giving advice to someone. We do not use this phrase when talking about ourselves. ex) Take whatever investors tell you with a grain of salt.

Sure, If said that, your listeners would understand. It's not incorrect, just not commonly done that way only because advice is normally given to others more often then we give it to ourselves :)

Absolutely. It is a perfect sentence. "With a grain of salt" functions here as an adverbial phrase that modifies "take". "When" is used as a conjunction to introduce an independent clause having its own subject ("someone") and verb ("tells"). The pronoun "it" represents the advice that "someone tells me".