Would it be really incorrect to write: 他们八点星期六吃晚饭 ?
Yes because in Chinese, when speaking of time you always go from largest to smallest. At least from what I know. I'm not a native speaker.
Yes, time adjectives go from largest to smallest. You would put year, month, date, time in that order if you were using them all in one sentence.
It's understandable but does not follow proper form: big dates like years, months, weeks to small draws like days of the week, time of day and exact time.