- I believe that the Quick Develop panel in the Library Module lets you make adjustments relative to the previous value.
- I don't think this is possible, resulting values from the Auto Adjust cannot be "burned in" and then sliders set to zero, I'm not even sure I see a point of doing this. (Maybe if you export as TIF or JPG and then re-import you can do this, but I don't really recommend this and don't see the point).
1. As in the reply by dj_paige - Quick Develop in Library module.
2. A 'round-about work-around' might be-
Adjust an image in ACR (as you would in LR), then create a PROFILE.
In LR, apply the profile and you have all the ACR adjustments showing and the sliders are set at '0' for further adjustment.
Create multiple profiles that will allow you to "burn in" from a choice of 'styles' before you adjust further.
Depending on the size of the image set you are intending to adjust (in your initial post you mention "all my pics"), it might be worth considering Lookup Tables.
That can effectively be done with a work flow from Photoshop -> Camera raw -> Lightroom
Excellent link above
That would give you the control I believe you are looking for Jan.
The other link I intended to include was-
This ZIP file includes the document with a Tutorial detailing how to create Profiles in ACR.
I was (am possibly still am) a 'Newbie' when talking about Profiles, LUTs, Cube files, etc, but the tutorial was easy to follow and I could easily create 'User Profiles' that appear in Lightroom.