At 6'1" you are the very upper limit of the 56cm, you would normally be recommended a 58cm frame.

Provided you want to adjust within a reasonably wide range you just loosen bolts and then slide the extensions along to make the distance from elbow pads to shifters longer or shorter, so no need to cut or disconnect shift cables. Its all quite easy to adjust.

The seat height is simply adjusted by sliding it up and down within the seat-tube. An alloy clamp secures it in position, so no cutting required. Use carbon assembly paste on it to prevent slippage. The seatpost has some useful measurement markings on it, so once you have found your correct height you can always go back to it. The seat clamp has a 2 bolt fixing with the rear on a thumbscrew to allow you to set the seat at the angle you require.
One thing to be aware of is that the Felt geometry charts shows this bike as having a two position seatpost. In actual fact they are only shipped with a single position one and it is in the forward position, so you may need to go up a frame size to get the correct toptube length.

SRAM and shimano use the same freehub spline pattern so you should be fine using these together.

I would suggest the 54cm. I'm 6'2" and found 54 to be really snug, and the 56 to be perfect, maybe even a little roomy

We can't test this but we can't see how there would be any issues with fitting these wheels to the Felt B2 R.

I can't say what size would be good for you, but I am 6'2" and found the 54 to be just a tad small, for you it will probably be too big

Sorry we can not change the spec on bikes.

One post + geometry.

We are unable to change the crank set here. If you wish to do so you will need to change the whole chainset rather than just the rings as compact and standard use a different BCD and aren't compatible.