"jedzony" has a continuous meaning - e.g. "My spider (was being eaten) by a sheep" Mój pająk (był jedzony) przez owcę. "zjedzony" has a more completed/finished meaning "My spider was eaten". So, in general, the Polish verb prefixes "z-"/"ze-"/"za-" add a completed/finished meaning.
Został is a form of the verb zostać.
The rule is easy. In the passive voice, use być for imperfective verbs and zostać for perfective ones.
In informal speech, być can often replace zostać in this context, however, zostać can never replace być (you would be understood, but that would sound very weird).
Well, maybe it depends on the region and for sure it depends on the quality (I'm sorry saying that) of your Polish. I know many people properly accenting "ę" and I can hear abslolutely nothing unnatural in their speach. Please, don't feel offended, but I felt obliged to write it here otherwise other learners might get misled
I have to interrupt. Pronouncing "ę" at the end of the sentence as "ę" is called hypercorrectness, which IS a mistake. Correct pronunciation is "e" BUT there are people who are able to pronounce "e" with nasality, which is something completely diffrent and is placed somewhere between "e" and "ę", AND it IS correct, maybe even more than "e", but not necessary. :)