What is "the current miner"? There is no one miner for each block. All miners are trying to solve a block for the same height, it's not like one is assigned to each block.

What prevents a malicious miner from making ridiculously large mempool-blocks? What if a malicious miner creates thousands of tiny transaction, say 1 satoshi each, and include 0 fee. Then he includes those transactions in a mempool block. How do nodes handle that?