Can all these groups be different and independent? Certainly, but no author has ever treated them as such or is likely to, because the differences in these esoteric groups are simply too nuanced, and honestly, nearly negligible. For the sake of convenience, I must admit I have chosen to ignore these delicate distinctions, and chalk up the differences to the whims of these opportunists at different time periods. The Emperor’s Pawns article combines the Emperor’s/Imperial (Interim) Inner/Ruling Circle/Council into a single entity under the Sarcev Quest entry.

Cracken's Threat Dossier states that the ERC had been formed only since the defeat at Endor, thus rejecting the theory that the "Emperor's Inner Circle" is but another name for the Ruling Circle, since the former existed prior to Endor. Furthermore, the DESB also refers to an “Imperial Ruling Council” and it is described as having several Dark Side Adept members, but again, despite the similarity in name to the ERC, reference to it is pre-Endor, and thus, it seems the Imperial Ruling Council cannot be the same group as the Emperor’s Ruling Council. However, since Palpatine’s Dark Side Adepts -- who make up part of the Imperial Ruling Council -- are (forcibly) loyal to him, and the Emperor’s Inner Circle was described as the group closest to Palpatine, and both of these organizations are associated with the same work (Dark Empire), and both are said or shown to have failed to take control of the Empire upon the Emperor’s death, it is highly likely that these two are related, if not the same.

The known composition of the Interim Council reflects the Empire's condition at that time. Norym Kim's power points to a serious weakness in the central government, as one of the Imperial Navy's raisons d'être was the eradication of piracy; given the known conditions of the galaxy throughout the post-Endor period, it is likely that he controlled one of the nominally Imperial regions of space in the Mid-Rim Region, near the Borderlands. Banjeer, Jeratai, and Immodet point to the triumph of Isardian stratocracy and the final collapse of the old doctrine of strict civilian control of the military, as they no longer report to the civilian-controlled bureaucracy or the Privy Council, but have a direct voice at the highest level of government, their continuing command of operational forces notwithstanding. Tan Starpyre's inclusion suggests timid steps toward liberalization, possibly to alleviate the effectiveness of Neo-Republican propaganda (notably, The Essential Chronology points out that the inclusion of Lord Manos, Ch'unkk, Kooloota-Fyf and the Prince Za was hoped to "give new life to the dying Imperial military" via "alien strength"). The influence of Lord Manos points to a breakdown in the Empire's traditional laissez-faire attitude toward labor relations, and suggests that the traditional alliance between the Empire and corporate leaders might have broken down. Much as the Mining Guild's dictation of trade policy is indicative of an astonishing collapse of the Empire's own industrial resources and its firm control of its corporate associates, the influence of the Lord d'Asta and the Prince Za points to an ironic relapse to the conditions of the Galactic Republic before the Naboo Crisis that prompted Senator Palpatine's election as Supreme Chancellor: private entities holding a de facto monopoly on shipping and trade routes that effectively neutered the state's power to regulate commerce and gave the monopolists the ability to directly and substantially influence state policy. This marks a complete reversal of the Imperial tradition of the boardroom being a junior partner to the throne room. It is especially interesting to note the shift in ideological composition; few if any of these Interim Councilmen have any clear political identity. The Interim Council resembles a technocratic government, and it is remarkable in itself that COMPNOR, the totalitarian wing of the monarchist party, is not represented at all.

Is it also analogous to "Von"?

If Fel was made a noble, but since he never used "Tan", that assumes that "Tan" is granted seperately from Imperial noble titles. (unless he was actually called Baron Soontir Tan Fel in any circumstances I'm not aware of)