Sharpening settings

2. Are there any general sharpening setting guidelines for working with portraits?

As a basic rule for portraits I'd suggest to set Details rather low (i.e.10) and Masking rather high (i.e. 70) - however, that heavily depends on your shot.

No, you don't have to undo them. Lightroom is a parametric editor, which means that all your edits are written in the catalog. Only when they are needed, such as when you export an image, those edits will be applied and always in a particular order that Adobe feels is best. So the order in which you make edits is irrelevant.