Q) What happens when two opposing players activate Romulan Commander during the same round?
A) If both players activate Romulan Commander, then it's up to initiative to determine which Captain fires first. If both players have the same SP and same ship Faction, then it's up to a die roll. If this happens multiple times during a game, then you roll the dice each time. It's different than the normal "roll at the start of the game and it counts for the whole game" rule because the situation generates "ex nihilo" each time it happens.
And of course, the Simultaneous Attack Rule applies when two Romulan Commanders shoot at each other.
https://dl.dropboxusercontent.com/u/30223400/Official.jpg CGuild: http://boardgamegeek.com/thread/1166819/st-aw-official-faq/page/1 (Captains #12)
Q) What happens when Keevan targets an activate Romulan Commander during the same round?
A) https://dl.dropboxusercontent.com/u/30223400/Interim.jpg As with 2 Romulan Commanders attacking each other, Keevan and RomCmdr are both attacking before each other. This constitutes a timing conflict, similar to if they had the same Captain Skill. Initiative would be used to work thru it and so the Romulan Commander would resolve before Keevan. Note that the Simultaneous Attack Rule would still apply.