There will be far less high grading going on this time around. Managers making the decisions will not want to reshuffle the deck again if they just did that a few months ago. Exceptions would be if someone was really just not working out or if someone new indicated they wanted a package. With us having gone through such an exhaustive HG just a few months ago, there will not be as much appetite for it now. Don't pin too much hope on being high graded anymore. Sorry, not trying to be morose, but if you think your position can be eliminated, plan to go with it. If you are laid off, you'll be in better shape. If you are not laid off, you'll be ok for a while longer while the company continues to try and hang on and you'll still have a resume that's ready to go. Suggest you read the post about making a layoff plan Lots of good info there that will answer other questions I'm sure you have. It's hard going through this for the first time. Keep your chin up.
As far as your second question goes, there may not be any cover off between the person leaving and the new person coming in. Always though, the person being laid off is notified before the person picking up some or all of their work is told. Sometimes there are arrangements that can be made for a handoff during the the 3 week notice period.
And your third question, high grading is always done in service to what the business needs are expected to be. Layoffs will affect people of all experience levels and all performance levels at this point. The obvious targets for layoffs are already gone. The people who management don't see clearly the way their peers might (the people you think are terrible but management does nothing about) are unfortunately just as safe as you are...because management doesn't see them clearly.
Good luck to all. And don't let yourself be defined by a layoff. You're so much more than that and you'll bounce back from all this chaos and be better for it.