Considering the meaning of the word Heme in our world and the background of Erik Granström (who once named a fjord Amygdala...) I get the feeling that the word should be pronounced in the same way you pronounce the protein hemoglobulin (technically hemoglobulin has a part that is called a heme-group (or sometimes haem-group) that has an iron ion), which would mean that the English pronunciation would be "heem".
In Swedish "Heme" is probably the most common pronunciation.
Anders has decoded my twisted sense of humour correctly and Ravenland (like my native setting Trachoria) is littered with ester eggs and references to this and that. Note that I do this mostly because it amuses me (and hopefully others), although in my novels the references often have some additional meaning.
So Heme is from the heme-group in hemoglobin as Anders says – the red stuff in our blood. I found this appropriate since the goddess is into blood sacrifices and is bound to the Iron god Rust (heme has an iron atom at the centre). I suggest the pronounciation ”heem” in English and ”heme” like ”stege” in Swedish. There's actually another blatant reference to heme in the game, but you haven't seen it yet. Let's see who finds it first. I promise a Bloody Mary to that person if we meet
It´s worth to note that the picture of Heme in the religion chapter is inspired by my friends work, the Swedish artist Vildhjärta ”Wildheart”. Go check out her stuff. It's really nice! I even have a sculpture of her here at home. http://vildhjärta.se/