The Epson L805 is a standard, low-end, end-user, inkjet printer. It is not capable of CMYK nor does it have a Postscript level 3 RIP. In short, it's nothing overly special and its not designed for use with high-end professional-level software such as Adobe Illustrator. It's a "mom & pop" printer, designed to print things an average user would print - ...
There is no standard.
If this is intended for office printing, I might be wrong but I don't think it matters much, as you can always check the 'Fit to Page' scaling option, which will auto-adjust the pages based on your particular printer and its particular margin settings.
I would use a minimum value between 5-8 mm and print with 'Fit to Page'.
No, there is not.
I have seen printers that leave only like 2 mm as borders, and some that need like 1.2 cm.
There are laser printers, inkjet, etc. So there is a loooooooooot of options. The biggest margin generally is the bottom one.
I would say that you need to learn how to interact with the printer. There are several applications that ask for the ...
Rotate the page content so it's vertical.. or set up a vertical document and then use the page display options to display the pages rotated 90°.
Basically, set up a vertical book with the content rotated. That allows you to use the "print booklet" command and get a landscape book.
Regarding scale for printers, don't worry about calculating area. 50% applies to dimensions. So a 70 x 40 print at 50% would just be 35 x 20. Always verify with your print provider, but this is pretty much a universal practice.