Regarding the Bible, you should read Young’s literal translation. It blew me away how much liberties that king James version took in their interpretation.
Is there a difference between '-th' and '-s' suffixes in English? I thought 'seeth' was just an old fashioned way of saying 'sees' but now I'm wondering if this is saying something about tenses that I'm missing.
-(e)th was the Middle English ending for 3rd person singular and plural present tense. It has mostly been replaced by -s in Modern English, except for a few fixed expressions (e.g. my mum used to say "quoth he" which is subtly different from the modern "he quoted" or "he quotes" which is normally followed by an object that is being quoted as well as the quote itself).
It's also interesting to look at the case system for ME nouns too, as it makes some things easier to understand. For instance the 's in Modern English for possessive is really just an abbreviation for the -es genitive case, which probably occurred when spoken forms had changed from -es to -s, which in turn was probably due to the shift to drop terminal -e from words which had started even by Chaucer's day (some places in Canterbury Tales you need to treat the -e as its own syllable to hear the rhyme, in other places it needs to be silent).
saith is to says as says is to sez, in the sense that each spelling is an attempt to represent English pronunciation in writing and their progression tells us something about how the pronunciation changed over time, but not everyone who uses a certain spelling necessarily also pronounces the word the way one might naïvely expect based on the spelling.
