If you don't like what the dealer's diagnosis is your only option is to go dealer shopping. this is still warranty right? If not warranty, and you don't agree, shop around.
There is a service bulletin for this issue....misfire when cold due to compression loss from sticky valves caused by fuel contamination. As a dealer its only smart to address this known tsb first. Sounds like there doing a good job. fuel contamination is why they replaced the injectors first...probably failed the active fuel injection test.