> Possible ways to highlite is to have a light grey: 
> <--> 
> » (at the beginning of the tab)

FWIW the latter (») is what I usually see used, with a lighter color to distinguish it from regular text.

> 3. Should tab space in comments and strings be shaded?  I think so.
> If so, should the shading match the comment/string foreground color?

Tabs in comments and strings should generally be avoided (\t can be used in strings), and at least for Python source I would assume they don't occur too frequently, so I wouldn't worry too much about this issue.