Ha! I'm picturing metal songs about weekly bathing, shearing sheep, and peaceful trade.
With the vocals of a black metal band, I wouldn't be surprised if some bands just make up shit like that and then apply darker lyrics to it when they need to submit the text to websites.
Lamb of God has done that with songs off of their first album. I can't find live footage that shares the same lyrics as each other or the CD booklet.
I think the growled and screamed vocals are a lot like backmasking, where you hear whatever the text says you're supposed to hear. It sounds utterly indecipherable until you look at the lyrics and then you're like "Oh yeah, I can hear that word fine!"
Not really. Yes, it takes some work to get used to growled/screamed words. However, listen to it enough and you'll be able to understand it without a transcription. Lamb of God, to continue the example, is a band I've listened to for so long that I don't need to read along to learn the words. Sometimes, when I first discover a new artist, I need some help acclimating to the singer's particular voice, but it's generally easy enough to decipher after one or two listens.
With that said, there are some genres where the vocals come out as grunts and shrieks, and I'll readily admit to not knowing the words (or liking those subgenres, usually). It is wholly possible that some such bands write lyrics retroactively, but my experience has been that the vast majority of metal bands write their lyrics first, then decide what kinds of vocals fit both the lyrics and the part of the song they're singing over.
Is it safe to assume you don't listen to much metal, or at least the styles that make frequent use of such vocal techniques?