Have election boycotts ever successfully undermined the results?

Answer 1

Yes, absolutely. A good recent example is the 2017 Puerto Rican Status Referendum. Citizens were asked to vote whether Puerto Rico should become a U.S. State, become a free entity, or remain a U.S. colony. All the parties advocating for something other than being a state boycotted the election for various reasons, so only 23% of eligible participants actually voted, resulting in an election where 97% of voters elected to make Puerto Rico a state. Elections with that level of agreement are very unusual in democracies, especially when there are more than two options on the table, so the boycotts effectively achieved their goal of invalidating the results of the vote.