Hi Carlos, let me see if I can answer your question...
1. As a matter of coincidence/ fact, I bought my fridge at the same shop. Answer: fact.
"As a matter of fact" is a whole expression. The "fact" in the expression may not mean actual "fact" because it's simply part of the expression. It's similar to how "de nuevo" in Spanish means "again" in English, and the "nuevo" doesn't have to be something "new." While there is nothing technically wrong with saying "as a matter of coincidence," English speakers don't really say that at all. When we use "coincidence" this way, we proceed it with "by." For example: "By coincidence, we have the exact same names."
2. The plan is a very good one, as far as it goes/ seems. Answer: seems.
The answer is "seems" because the plan has not been carried out yet, so there is still doubt about how the plan will actually go. "As far as it goes" is only used if something has already happened (wholly or partially). For example: "The evidence for global warming is convincing as far as it goes." Here, we already have lots of evidence to convince ourselves of our finding.
I hope I helped answer your question. Please let me know if I haven't or if something isn't clear. Thank you.