To me yes, perfectly. And yes, "Target_image_noise_level__error" is an alias for "adaptivity_targetError". This is the property line you get when you list all Corona properties, sorry if that was confusing. Nothing old/deprecated in here:
.adaptivity_targetError (Target_image_noise_level__error) : float
And as I understand, you should be able to see and check your noise target setting in the mentioned "Plugin Info Parameters pane" aren't you?
As for progress: You seem to be more interested in how much time is left, the actual noise level is secondary. ETA calculation is not an easy task when noise level as stop condition is used. A standard progress bar from 0 to 100 is only possible to do with access to the internally calculated ETA imho. This is why I would love to see (maxscript) access to ETA because you cannot do this easily by yourself - or any external application like DL.
But according to Mike O., Coronas ETA should be displayed in slave Task Render Status like "Rendering pass x (elapsed: y, left: z)".
I cannot check that all because as mentioned I have no DL installation available and never used it, maybe someone having DL could take over. But to me it seems that you already have all information you like to see, even if possibly access to it could be dressed up a bit.
Good Luck