What exactly is the “Merkle tree” often mentioned in exchange reserve proofs?
My understanding: When a block contains thousands of transactions, proving that one of them is included only takes a dozen or so hashes—you don’t need to download all the transactions. This is especially important for mobile wallets.
If any transaction in a block is changed, its hash changes, and that change propagates up to the Merkle root. The block hash changes too.
My current practice: Light wallets get their data from the node they’re connected to. For large incoming payments, I cross-check them using several block explorers.
The header of every $BTC block contains a Merkle root that summarizes all the transactions in that block.

Personal notes only, not specific buying or selling advice.