Misconception 2 : Only one user can interact with a smart contract per block/transaction.

Also not true; the point of contention is around the UTXO, but many UTXOs may be governed by the same smart contract.

This fundamentally comes down to the shift in thinking from Ethereum, where you call into a smart contract to make it do something, and Cardano where you lock outputs with a contract, which determine when they can later be spent.