Moddan father of Helga (et al) has *no parentage assigned* in Orkneyinga Saga - and they were very, *very* big on ancestry when it was known (they'd go on for pages sometimes). He may have been a wealthy farmer with a lot of land, but clearly the only "nobility" to him was that of exalted character, and not of bloodline.
Helga was not Jarl Haakon Palsson's wife, she was his mistress - which clearly implies that she was of lesser rank and no Jarl's daughter herself. There would have been bloody war over the insult otherwise. (A farmer, be he never so wealthy, would on the other hand have been *flattered* to have a Jarl take such an interest in his daughter. Other times, other morals.)
Identifying Moddan's otherwise-unknown father with "Mutatan or Muddan" from earlier in the Orkneyinga Saga just won't work. Even if this were a real person, Thorkel Fostri made mincemeat of him in or before 1040 (the "Tuatha de Danaan" had nothing to do with it, and Orkney had everything to do with it - Thorfinn the Black wasn't going to let "his" Caithness be taken away from him by anyone. Caithness remained an Orkney possession until long after Thorfinn's time.)