... in Anime. ... I've read online that ておらん is an overproud version of 食べていない.
As you say you heard it in anime, I suppose the おらん was used as 老人語 (old people's speech), which is a kind of 役割語 (role language).
The ん originally is an old negative auxiliary ぬ. It's attached to おら, which is the 未然形 (imperfective form) of the subsidiary verb おる.
It was translated as "I didn't eat your soul".
Yes... ～ておらん, or ～ていない in standard Japanese, can mean "didn't do~~" or "haven't done~~ (yet)", as well as "is not doing~~ (now)". See this answer for detail.
I'd like to know how should Oru/Oran be attached to U-ending verbs
You can just attach おる/おらん to the te-form of verbs, eg:
する -- しておる・しておらん
くる -- きておる・きておらん
いく -- いっておる・いっておらん
おちる -- おちておる・おちておらん
おしえる -- おしえておる・おしえておらん
and what's the past form of ておらん. I read the past form of ておる is ておった.
The past tense form of ～ておらん would be ～ておらんかった. (But here in your example it's already past tense, so it wouldn't have to be お前の魂は食べて(は)おらんかった.)
As a side note: The は in 食べてはおらん is the contrastive particle は.
食べておらん and 食べてはおらん are both correct, and mean almost the same thing. The latter might sound a bit more emphatic, or might imply that they didn't eat it but did something similar.