As a shield drone always has the same T and save as the model thay are protecting they can always have hits allocated to them before the model they are protecting.
The problem here is that you intend to allocate good
AP wounds to to a model with an
inv save.
As far as I know there is no good way to deal with situations like this, the best way I have found is to use the mixed armour rules on page 76 as a guidline, those rules are however not designed to take into account a small unit with the same save but with mixed in
inv saves.
As its TAU we are talking about I am asuming the 5 hits are from shooting, if the commander and 1 shield drone is in
CC then its change of pants time in any case.
Firstly, no matter any other rule the shooting player will be able to call a "torrent of fire" (p26) hit on ither the commander or the drone, this hit has to be saved by that model and if the save fails that model will have to suffer the hit, it is however upp to the defender wich of the 5 hits is actually allocated.
The smart bet here would be to allocate the hit to the drone, this would meen that the defender takes one of the hits he needs to use the
inv save against on it but if that fails then the drone is gone and the remaining 4 hits will all go on the commander, one automatically wounding.
for the remaining 4 wounds the defender can divide them as he sees fit, I would follow the p76 rule that each model be allocated 1 wound before anyone can be allocated a second.
Once the wounds are allocated the saves are rolled, the models can choose to use
inv saves and thus the drone can end upp taking both no save shots.
Any wounds not saved meens you have to remove one wound from the unit untill all wounds are accounted for, this does meen both
inv saves can be failed by the single drone and the commander suffering a wound because of it.
Yes, you can get very odd situations when the model being removed does not match T and save rolled against.
In reality each step in shooting is completelly independent of all the other steps.
The gist of it all is that you missed the torrent of fire rule but were correct if we ignore that