Not speaking as a grammarian, but simply as a native speaker of (American) English, the sentence parses correctly, but seems somewhat stilted. "I have known her since 2012 ..." sounds completely natural. "I have known her well, since 2012, .." sounds natural, with the comma separating it into two thoughts, "I have known her well" and "I have known her since". Absent the comma, expressed as a single thought, it just doesn't sound right. In either case, the adverb "very" doesn't add anything. It's a general rule of writing, in English, to be sparing with adverbs. Generally, if the verb you have chosen doesn't sufficiently express your meaning, choose a different one. A properly placed adverb can be very effective, but littering your prose with extraneous adverbs serves only to muddle. https://www.writingforward.com/writing-tips/avoid-adverbs "The word run is a verb and the word quickly is an adverb. Put t yt?hem together and you get run quickly. But run quickly is better stated as sprint."

Yes that is correct. We use SINCE to talk about a specific time [year, month season etc] while we use FOR to talk about length of time [number of weeks, days, months etc. So you can say I have known her very well SINCE 2012 or I have known her very well FOR 5 years. :)