a. What dpi resolution is needed for decent scans of 6 point text? (Old
newspapers use smaller type)

150 will do, you might try 200

b. What is the best way to process the grayscale image down from 256
shades to monochrome?

Don't. Leave it grayscale, or you loose too much info. Maybe you can size
the files down by going for e.g. 64 grayscades, you can try.

c. Is decent quality OCR possible on a gray background newsprint 6
point text at anything below 300 dpi?

Again, try. Automatic contrast settings for most scanners do a decent job,
but the question is too general for a specific answer.

d. What is the best way to scan in pages larger than your scanner
(e.g., 11x13 inches on an 8.5x11 inch flatbed scanner)?

Dunno.

e. Is there a better way to get straight scans? I line up the edge of
the paper with the edge of the scanner glass and close the lid but keep
getting scans tilted. This is troublesome because old newspapers are
usually not printed exactly straight horizontally or vertically due to
bending of the paper during printing.

If you're willing to buy a sheet feeder and loosen the leaves of the
magazines, you could try that. Otherwise it's a lot of manual labor.

Good luck
Jan