I don't think there would be any difference in their communication from being stationary from their points of view.
This sounds right. I think there would be blue/red shift on any radio communication if the ships are still accelerating or decelerating, just because the radio waves would have a constant speed but the ship would be accelerating or decelerating.
Even though space is a vacuum, there is still something there, and your communication is going to have to pass through it.
Im gonna get sloppy with the math because its 12:30am here.
You're traveling at 149,896,229ms and want to do a 1 second burst transmission on your 1m wide antenna, over a distance of 50,000,000m. That transmission will be spread over a volume of about 749,481,150,000,000m3. Deep space has an average density of maybe 1 atom per cm3 but might be as high as 1000 atoms/cm3. (This is where I know Im too tired for this) because I converted it to 749,481,150,000,000,000cm3, the most common element in the observable universe is hydrogen with an atomic weight of 1, that equals 1.6735 x 10^-24 grams, or as I like to type it out 0.0000000000000000000000016735g. If you take the next steps you're trying to transmit through anywhere between 1.25mg at the high end and 0.001mg of hydrogen on average. Hydrogen, although flammable, is actually capable of insulating against radio signals, so you're going to have signal degradation just due to the amount of particle you're passing your communication through. How much radio signal can 1mg of hydrogen block? I dont know. Maybe all of it. Maybe none of it.
Im not sure if any of that is right, but it sounds good to me right now