I was following you right up till this bit:

By the way, as previously noted
    d[1, 2]
    d[(1, 2)]
are at present equivalent. However, in the new syntax
    d[1, 2, a=3]
    d[(1, 2), a=3]
are not equivalent. (The first has three arguments, the second two, the first of which is a tuple.)

In the "New syntax", wouldn't these examples map to:

d[1, 2, a=3]  =>  d.__getitem__((1, 2), a=3)
d[(1, 2), a=3]  =>  d.__getitem__((1, 2), a=3)

I.e. My understanding was that the existing conversion of all positional parameters in a subscript would be packed into a tuple (strictly for consistency with legacy behaviour) while any keywords would be passed as kwargs?