True, that's why you get the current miner to order them in a blockchain. So then they become ordered.
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?
The last miner to mine a block can generate mempool blocks until a new miner comes along == the current miner.
As for the malicuos miner, there could be block size limits placed on the mempool blockchain just as is done on the core blockchain. But we would probably build the flexibility in from the start so that we do not repeat the current issue with the block limit. Yes, malicous intent needs to be addressed, but I think we can work the details out for that issue prior to implementation. In general it i not in a miners best interests to mess up bitcoin...
Thanks for the comments :-)