When I import files from my SD card to LR6 it undergoes RAW conversion by the LR/Camera Raw engine.
When you import from your SD card to LR6, the file is still RAW. LR/ACR will generate a preview but will not change the original file
I If I then process in OP11 as a LR plug in, does the RAW conversion occur from the original file and if so, when I re-import to LR, does the file remain a .DNG (raw) file? I ask this as a .DNG file would be preferable as white balance and other adjustments can still be made or am I mistaken?
A follow up question is once I am in the Optics Pro plug in and adjust the file using a Film Pack preset or perform other adjustments and then re-import back to LR6, does that file remain in .DNG format?
When you use the OP plugin in LR you do get to open the RAW file in DxO, so the RAW conversion will be performed in DxO. You can then chose to output to DNG in DxO, but this DNG is not a Raw file, neither a completely processed file as a Tiff or JPEG. Some people refer to this as "half baked".
The DNG that you get out of DxO it is called "linear DNG, meaning that demosaicing has been performed, as well as any optical correction & edits, but it has not been encoded to a color space such as AdobeRGB, sRGB or similar.
You can open this "Linear DNG" in LR and be able to perform White Balance, Assign a DCP camera profile (you can even create your own custom profiles out of it, if you use a color checker passport with the profile creation tool).
There is a small issue or weird characteristic: If your image has blown highlights, then you should perform white balance in DxO, otherwise you will get non-neutral tones in those blown out areas when performing White balance in LR on the linear DNG.
Note: When you reimport the DNG to LR, you might see a dull image (may users report this). This is due to the assignment of a DCP profile that might be different than your preferred one. Just change the DCP camera profile in LR to your desired or custom profile.
A follow up question is once I am in the Optics Pro plug in and adjust the file using a Film Pack preset or perform other adjustments and then re-import back to LR6, does that file remain in .DNG format?
Yes, if you choose DNG output from DxO, the same as the answer in the previous question. I'm not sure if it will keep B&W conversions.