Each inversion can have the middle and upper notes exchanged (there is no rule about which note goes where, only how to interpret the bass note and the notes above it
The figured bass tells us the interval distances above the bass note and is always referenced back against the bass note and the current scale. The intervals are also reduced to within the octave, which is why the alternates for eah of the above chord inversions are shown as they are for the simple versions.
I never looked at it that way but you are right. It is an inversion of the D major chord. But for chord naming purposes can't you name it two ways? If this chord was to be named on sheet music with the chord chart it would be either D/A or Asus...
How would you name it from an A major perspective? The 4 replacing the 3 makes it a suspended 4th. But I don't know what to call it after that because of the 6.
You wouldn't try to name it as some form of A chord, when you have all three notes of the D major chord present - no need to complicate matters D[sub]/A[/sub] is the only way of naming it in chord symbol notation.......