If I apply early decision to Tufts (deadline Nov. 1st), but I get denied/deferred, can I apply early decision II to William & Mary (deadline Jan. 1st)?
If and only if you get rejected/deferred from your first ED (Tufts) school before the ED deadline for the second school (W&M) then it should be fine. I would carefully read the wording and check the dates of the ED plans of both colleges just to be certain.
Yes. If you get a “no” or a deferral from Tufts, you can apply ED2 elsewhere. Should you get a deferral at Tufts and should you apply ED2 to W&M and should you get into W&M, you will need to withdraw your app from Tufts. Most schools defer, rather than reject, tons of ED applicants, so you need to figure out how you feel about moving on while there is still a possibility of admission at your ED1 school.
Yes, that is the whole point of ED 1 and ED2.
As @gardenstategal pointed out–you can apply ED2 if you get deferred from ED1. However, if you are accepted ED2, you must withdraw your application to the ED1 school that deferred you, thereby eliminating your opportunity to see if you would be accepted RD. Make sure you are willing to do that before applying ED2.
Yes, only if you are rejected/deffered can you apply EDII. If you get deffered from EDI, then you will be simply considered the same as RD and you must attend your EDII school if accepted.