Split coins in wallet using coins from coinbase.com
coinbase.com claim their sends have replay protection: "You can safely send any BTC or B2X from Coinbase without worrying about replays on the other chain."
  Does this imply that the coins I get in (1) can be used as a "taint"?


Yes, if a transaction only exists on one blockchain, it is by definition only valid on that blockchain, so any other transaction which spends from it is also only valid on that chain. 


  Is this necessary, or are my BTC coins immune to replay by virtue of already having been spent?


The latter, since your outputs have already been spent on the B2X chain, any replay of your BTC transaction would be rejected as a double-spend there, so you can safely just spend your BTC normally.


  Now that I have a wallet for each chain, will they remember which chain they belong to? 


I assume that would be the case, but I'm not familiar with the wallet so I can't answer this.

I'm unsure what you mean in your last point, send which transaction back to coinbase?