There appears to be ambiguity about which of these was Jarl at the time of the Markarth Incident, although I suppose the answer is "neither" as Madanach had ousted them.

Did the Forsworn kill Hrolfdir as part of their seizure of Markarth, so that Igmund succeeded him without actually having control, and then recruited Ulfric to help get the city back? Or did Hrolfdir ask Ulfric for help, and Igmund succeed his father after Markarth was recaptured? I suppose there's also a possibility that Hrolfdir was killed during the two years the Reachmen ran the city, but that period is described as "peaceful", so I doubt it.

The significance is that one or other acquiesced to the Imperial demand to arrest Ulfric. Was that Igmund himself, or his late father?