You raise a lot of interesting issues I ll give my thoughts on some of them First I think you re right that Eric is at least in some sense more readable Second you re right that we have article titles like Eric I of Norway and Eric VI of Sweden but also Eirik II of Norway so it s not quite consistent I think that a high level of Anglicization is expected for the names of royalty where familiar names tend to be used if at all available If Eir kr H konarson had ever called himself king I would be fine with having his article at Eric II of Norway But he wasn t quite royalty and he is a somewhat more obscure historical character than the kings so I m not quite sure how to apply this As for medieval usage I haven t made a complete study but what I ve seen so far is that the Latin charters witnessed by Eir kr during his life have Yric dux most commonly or Yrric dux or Iric dux The Encomium Emmae written in Latin in 1041 or 1042 has the form Eric and titles him as dux et princeps provintiae quae Nordwega dicitur The Anglo Saxon chronicle written in Old English of course has Irke a dative form or Yric or Yrice a dative form or Eiric Maybe I missed a variant Old Norse texts are usually published in normalized orthography where the name is Eir kr In diplomatic editions you will probably find forms like Eirikr or Eiricr or Eirijkr but I don t have any at hand at the moment so don t take it as gospel I ve got to run now will address the rest of the question later I m actually more worried about the patronym