Welcome!
I think sometimes people are too quick to makes things binary along the lines of “either it is objective literal history, or it is purely a symbolic/allegorical/fictional narrative.” I don’t think that does justice to the way history was told and transmitted in the ancient world.
I think Genesis 1-11 is Israel’s history. But their conventions for telling it were different, and I don’t believe there is a surefire method for separating out objective historical “facts” from the embellishments, didactic lessons, identity-forming bias, and cultural presuppositions that are inherent in it.
What did Abraham mean by saying he was over a hundred? Maybe he had dutifully counted the years of his life, or maybe their culture had different ways of assigning ages. Many cultures of the world don’t actually count yearly birthdays, you identify your age by what season of life you are in and your social status in the community. I think the demonstrable numerology in the recorded ages of the patriarchs shows that more was going on than simple counting. But I don’t think that just because you can demonstrate that Methuselah wasn’t literally over 900 years old, that doesn’t mean that he is automatically a made-up person who never existed.
I think this is a chicken and egg question that you won’t find definitive answers for. The version of the account of Genesis we have (which most scholars think was compiled or at least edited from earlier sources and oral traditions and put in its current form post-Exile) was composed or at least arranged and redacted long after Sabbath observance was a Jewish cultural phenomenon. So did the very first accounts of creation cause the Sabbath observance, or was the Creation narrative artistically worked out to support and validate Sabbath observance and draw metaphorical connections for a community that already saw a six day work week as normative? I don’t know that we have the evidence to prove either way.
A lot of how you approach these questions boils down to how you see inspiration and authorship of the Bible. Most fundamentalists, with adherence to plenary verbal inspiration and inerrancy, see the Bible as dictated to people by God, or something close to it, and envision Moses sitting at a desk and recording whatever God told him. Most Bible scholars, even fairly conservative Evangelical ones, have a different view of both inspiration and authorship. Here’s a good Mike Bird post on that. https://www.patheos.com/blogs/euangelion/2018/07/the-case-for-the-dynamic-view-of-biblical-inspiration/
Well, they talk about them as if they are real people. I would quibble that doesn’t necessarily entail that every thing written about them in the OT is factual in our modern sense, or that every reference to them is to historical facts about them. They were also well-known literary figures in the Jewish culture. I can refer to Julius Caesar as a real person even if most of what I know of him comes from Shakespeare’s imagination. I know the analogy is not perfect because we attribute authority to biblical history, but again, some of these things presented by some people as logical entailments depend on presuppositions about inspiration and inerrancy.
You might be interested in this thread, as it dealt with some similar themes: Jesus Mentions Adam & Eve = Evidence, A "Mistake," Or Accommodation