It doesn't matter whether the data format in this case is a Merkle tree or a flat CSV file. OK, we add an extra column in the CSV, now we have "secretvoterid, vote, signature", still no need for blockchain.
Also, you are still fundamentally trusting a centralized authority - the government who issues the keys, and the same government you were worried about fudging the vote count. Maybe the government is adding people who don't exist (sybil attack), and the "signatures" on some of those votes are just random noise, how do you know? So your scheme gains you nothing.
At the end of the day, the only trustless scheme for voting is being able to tie votes back to names so that illegitimate votes can be challenged by the public. Any other scheme requires you to trust the government to count votes fairly, or issue keys/voter tokens fairly (we can call these a "ballot"). Blockchain doesn't change this, and if we want to get rid of the secret ballot we don't need Blockchain to do so.