In chronological terms, they are the same if said during the same week. "...this coming Friday..." refers to the next occurring day that is a Friday. If they ONLY say "Friday" without stating which Friday. We do not exactly know when. It would depend when they made that statement. If the engagement is very important, if would be foolish not to have the speaker clarify "which" Friday they are speaking of so that there is no confusion. This is especially important when speaking internationally. If we say Friday, the day we think we are speaking of may be a Friday in one time zone. However it may be a Saturday in another time zone. It is important to state which Friday specifically. Friday, European time or Friday, California time?

They are essentially the same. These sentences are examples of two of the three ways English can show the future: 1. will + (verb) = "They will carry out..." 2. present continuous = "They are carrying out..." 3. (to be) + going to (verb) "They are going to carry out..." There can be nuanced differences between these, but they are mostly interchangeable.

You can always use a) will-future instead of b) present continuous, when talking about fixed future arrangements. a) is a more formal usage and so might be more appropriate in some contexts.