Does this kit include the o-ring?

It's included and preinstalled on the fuel pump.

Why doesn't this include the gasket? Is it not needed? And or does it include just the o-ring?

Yes, it is recommended to replace the HPFP gasket as well. Please refer to p/n: 11127588416 for the gasket accordingly.

Will this work on a 2011 135i?

No, this does not fit N55 135i models before 3/2012. We answered this in a previous question to a degree but any N55 before 3/2012 uses a rotary style high-pressure fuel pump that is driven off the vacuum pump under the intake manifold. As a 2011 model year vehicle we have to assume your vehicle is built before the 3/2012 production date split so your vehicle uses p/n 13517616446 as the high pressure fuel pump. You can find it listed on our site for a 2011 and 2012 135i's up to 3/2012 production date split.

So, this kit is to update an earlier N55 that uses the N54 style rotary HPFP to the later N55 piston style HPFP?

No, this kit doesn't change the type of high pressure pump installed at all. There is much more than just the pump if the switch were going to be made. If the vehicle is equipped with the rotary style pump it can only use that style pump unless several other components are swapped out including the ECU and engine wiring harness. This pump is for early N55 engines that have a different factory wiring harness and also use this tappet style pump as the connector is different on the pump.

Do I need the adapter in this kit if my manufacturing date is 12/2012?

Yes, you would need the adapter. This is specifically put together for vehicles that would not have the adapter already installed from the factory.