As I see it, the only lens correction that is advantageously done before demosaic is CA.
1. scaling the red and blue channels for CA means that data is already compromised and resampling those channels twice is likely worse than the gain of not resampling the green channel(s) prior to Debayer,
2. at an early stage, resampling is probably close to transitive in the sense that
DEBAYER-sample(DEBARREL-sample) == DEBARREL-sample(DEBAYER-sample),
3. resampling the raw-data influences moiré, and thus influences moiré corrections that may be present in the debayer stage,
Having said that, one might discuss the exacts of the internal algorithms, but the question remains how this should be represented in a DNG file. And also how this should be represented in the GUI.
If you think of a DNG file as the RAW data + parameters for interpretation, then it becomes very apparent that creative edits are not initially relevant for RAW conversion. Neither is interpolation of two colorprofiles.
what should actually be exposed to the user and more importantly HOW. We now have a hefty 9 sliders in the detail tab of LR, but is there a reasonable method, set of steps, that will get me to the optimal result most efficiently? Is there a single optimal setting?
If i have found an optimal setting in the detail section, will adjustments of the tonal section adversely affect the detail settings
? It seems the detail setting is now very sensitive to small changes. I find it very difficult to come to a reasonable setting.
By reasonable method I mean something like:
1. set all sliders to default,
2. start by adjusting slider A until you see this,
3. then adjust slider B until you see this,
4. if you see this, then adjust slider C, otherwise leave it untouched,
Similarly, there are some noise reductions that may be useful prior to debayering (and lenscorrections), like dead pixel elimination. If these corrections are defined properly, they translate easily to a DNG definition, as well as to a proper GUI and processing method. If you see a smeared pixel, first try the dead-pixel elimination button/slider in the debayer section. If that doesn't change it, use whatever other section the raw converter producer thinks appropriate. etc...
Positional problems with a parametric brush doesn't seem to me to be a very good argument to hamper proper implementation of RAW-conversion features and standards. Lenscorrections seem more intrinsic to raw-conversion than parametric brushes...