Well I can only explain the Rebel's Diplomatic Standings since my main character is there, I do have a Volkhan-sided character but it's only used to even numbers during events. Else I'm almost 90% of the time playing as Lukas Wexler.
The Rebel's Diplomatic Standings is pretty simple as they are a copy-paste of the former SCRA diplomatic standings.
I cannot comment on the way Volkhan's regime operates as I'm not experienced in that area, but I do know that they retain basically the same goals as the former SCRA but the methods to achieve them are more brutal, more violent and more aggressive. Where Katz would focus on agreements with different parties to further the Coalition Movement, the Volkhan would simply use direct force against entities that disagree with him. At least that is how I have perceived their recent actions, but please note that I'm not an expert on this side so don't take my word for it.
It also makes sense given Volkhan's initial "propaganda" message that was directed at throwing mud at Katz for his diplomatic failures. Claiming he had grown soft forming too many deals that divert the Coalition's attention towards fulfilling diplomatic promises rather than strengthening the movement's position in Sirius.
Perhaps this has given you a bit of an insight as to why they are hostile to a Faction such as the IMG whom were in good terms with the former Coalition Establishment. Perhaps Volkhan understood that you were "in good terms" with the Rebels and wanted to continuously attack your Faction in an attempt to damage your relationship with the Rebels thus making it that much harder for the Rebels as they depend on every ally they can get. A wise move I would say.
Now going into specifics about the Dublin System, I guess this is just related to the Rebels current location, they come and go from Bretonia, but Dublin is also directly connected to Omega-49 and Omega-50 where a lot of the current action is going on between the Loyalists and the Rebels. Volkhan's Forces utilizing raids against the Bretonian House are most likely for two reasons, the first being to of course forward their movement through the means of direct force: "Surrender to the Volkhan or face dire consequences." as well as trying to root out Factions that might harbour the Rebels.
Finally, as for the definition about real Role-Play, well bringing the Reavers as an example again, their "real Role-Play" is about hunting targets and fulfilling hit-contracts. That has almost entirely to do with engagements, the Volkhan follows a similar aggressive approach but is after completely different goals. How can one be validated when the other is not?