How would these parameters affect mining and the size and content of the mempool? Q1 and Q2 are theoretical questions to confirm my understanding.  I know doing the following serve little practical purpose.

Q1: If a mining pool were to set blocksonly=1 in their bitcoin.conf, would all blocks created be "empty blocks" (only the coinbase transaction)?

Q2: If a mining pool were to set mempoolminfee very high, would all blocks created be "empty blocks" (only the coinbase transaction) or possibly only a few transactions?

Q3: I can't seem to find the syntax to put mempoolminfee in bitcoin.conf.  Any help appreciated.
Q1: If a mining pool were to set blocksonly=1 in their bitcoin.conf, would all blocks created be "empty blocks" (only the coinbase transaction)?


Yes. The mempool would be empty so the miner would not have any transactions available to add to a block except for their own coinbase transaction.


  Q2: If a mining pool were to set mempoolminfee very high, would all blocks created be "empty blocks" (only the coinbase transaction) or possibly only a few transactions?


Yes.


  Q3: I can't seem to find the syntax to put mempoolminfee in bitcoin.conf. Any help appreciated.


The option to set is actually minrelaytxfee. This is because a node will only add to its mempool something it will relay, so the minimum relay fee is what controls the minimum feerate to be added to the mempool. Note that this is a feerate, not an absolute transactionfee.

The documentation for this option is:

  -minrelaytxfee=&lt;amt&gt;
   Fees (in BTC/kB) smaller than this are considered zero fee for relaying,
   mining and transaction creation (default: 0.00001)