your point about the "middle ground" between control and indifference is the point at which the defenders of the PD in its most extreme form lose the argument.
Because basically they're defense of it DEMANDS that they see no difference between intervention to help and domination or conquest. In the real world we'd recognize this as nonsense, but really to "defend" actions like in "Dear Doctor" leads one to poorly thought out arguments like either:
"giving them the cure is interference, and interference leads to conquest and subjugation!"
"giving them the cure would mean going around having to "help" civilizations everywhere in the galaxy, which would require such a drain on resources and time it would be unworkable!"
both are basically sloppy versions of the "slippery slope" and shouldn't really be taken seriously.
Help is not synonymous with imperialism, nor does it require crusading around the galaxy in an endless quest to right all the wrongs out there.