It's a student guide written for other undergrad anthropology students, so we should read it in that light and make affordances for imprecise language and a lack of generality.
In that light, what is it describing postmodernism for? Simply put, it's a [set of] frameworks through which to analyze the kinds of things anthropologists work with. This an important because many of the issues you're seeing are the direct result of trying to extrapolate beyond this academic heritage.
So from an academic anthropology standpoint:
1. Is definitely not flat earth silly. To give some context, prior to PM in the 70s, anthropologists were doing silly things like measuring the amount of food on the table every day and how many knives were in the cupboard. That's nice quantitative data, but it doesn't really tell you much about culture or human behavior. Thinking of language and text as fundamental aspects of culture and behavior is much better, though I personally have some quibbles with that focus.
2. Specific to anthropology and a bit of academic vogue here.
3. Remember that this is an academic framework for analyzing academic things. In anthropology, sometimes people all see the same physically impossible thing, or see different things despite observing bthe same physical events, etc. To deal with this, sometimes it's useful just to view things as if people are in different realities and observing different things. It's not a claim on the real underlying epistemology.
4. Virtually everything academic from the last 40-50 years has been influenced in some way by post modernism. Bit of a Seinfeld effect here.
5. They have a section later about methodologies of postmodernism. Yes, it's a bit ironic.
6. Same as 4, but you're right that it's not unique. Doesn't have to be.
7. This is specific to academia, which was (and largely still is) made up of very much Western derived institutions. Specifically, this is about things like "maybe the structure of our institutions leads us to certain types of research design and valuing certain types of information over others" and "maybe other ways of organizing academia / research / knowledge might also lead to useful results". It's not to suggest the only target for criticism should be Western institutions, merely that they're not the only way to do things and might be improved.
As for whether the ideas of PM are being miscommunicated, sure, I'm open to the idea. But there are also millions of people in academia who do get it. Since we all know academics can be remarkably stupid, it's not an intelligence gap. Maybe it's just something that doesn't make sense without practical experience in the kinds of problems it's designed to help?