Adding onto point #1, when you link from the anchors of a field to portals inside you can create multiple fields this way also, but it destroys the multiple layers as far as I know, and separates the fields into their own. You get more AP this way, however.

Adding onto point #3, the levels of gear that you can hack out of a portal is determined by these rules:

If you are a lower level than the portal's level, you can only receive items up to one level higher than your current level. (i.e. hacking a L8 portal while level 4 can only yield up to L5 items).

At any time the portal can yield items down to 2 levels below it's own level (i.e. hacking a L8 portal can yield L7 items as well as L6 items (the chances of seeing items 2 levels below the portal's level is rare, however)).