1. Can I adjust white balance for .jpg files in LR? OR does it really need to be done with a RAW format file?
Yes, you can adjust white balance on .jpg files. There will be two differences. For a raw file the adjustment scale will be along an absolute temperature scale so you can set, for example, an exact 6500K white balance. For a JPEG the adjustment scale is relative because the white balance is already baked into the file. The other difference is that with raw, you will be able to push white balance much further from the current setting before the image starts to fall apart.
2. Can I adjust exposure compensation for .jpg files in LR? OR does it really need to be done with a RAW format file?
Similar answer. You can adjust exposure up and down for a JPEG, but you will find a much narrower range of adjustment before the image visibly degrades. It will be easier to darken the image than to lighten it. When you increase exposure on a JPEG, shadows you lightened will look much worse a lot faster than with a raw file.
3. If I can fully adjust the white balance and exposure compensation in LR for a .jpg or RAW file, is there any real benefit to doing it in camera? Will you achieve the same quality level doing it in LR vs. in camera? Are there any downsides to doing it in LR vs. in camera?
The problem in any editor (not just Lightroom) is that your files, raw or JPEG, have been limited by the dynamic range of the sensor in the camera. You can't "fully" make adjustments if the camera can't "fully" record the scene in the first place. If you want to make a +4 EV adjustment to an image and you do it in camera, the image data is in the sweet spot of the sensor and it's going to look great. If you don't adjust in camera and you expect to make the +4 EV adjustment in an image editor, you are going to try to push a lot of shadow data up into the lighter tones. The shadow data is the lowest quality, so lightening it will reveal noise and banding. In addition, if your camera doesn't have enough dynamic range, the camera might not even record down far enough for you to pull off a +4 EV adjustment in software. The better sensor you have, the more likely you can make big adjustments and like the result.
Raw just gives you more room to make mistakes. It is always better to try and get it right in camera.
jpg files wb can be adjusted but more can be done with raw
I general leave camera wb on auto or sunlight and do any adjustments in LR
if you capture with raw+jpg it would pay to adjust the camera wb just for the jpg files. Something I read yesterday re: photographing with B+W image in mind. Set the camera to mono/B+W and the jpgs will be b/w but the raw file will be colour. [You might need to use live view to see the B+W]
I would suggest to use raw only so you are not worrying about 'internal' settings and then have to remember to reset them. RAW will give a better file. You could make an import preset that will make the raw files look more like an off the card JPG. Add some contrast>>reduce h/lights>>reduce shadow>> add some clarity>>add some vibrancy>>add lens profile>>maybe some sharpening/noise reduction and half your PP is done on import. A way to work out what 'some' would be to capture raw+jpg fine standard and adjust raw files to sort of match the camera jpg . Pick a sort of average and make a preset to suit.
That make sense?
Bit off Topic
Another clever 'auto' setting for the camera is to make a custom setting so when grab the camera it's ready. My 5d11 C1 is something like....... Av....evaluating metering....auto ISO....auto WB....raw.....f10......single shooting...one shot AF.. Yours would depend on your 'normal' subjects. When not using the camera I just leave it on C1