I think this thread has been dead for many years but it is baffling that no one was able to actually answer these questions..
tennis_hand said it was a "useless question" yet amateurs tend to not understand grips and when they should be changing. That leads to players using the same grip for quite literally every shot which helps lead to shoulder surgery and tennis elbow. So his question is VERY relevant since we are talking about tennis.
1.does Federer holds the racquet in continental grip while awaiting the ball,
Yes, a pro would commonly hold the racquet in a continental "ready position". After Federer strikes the ball and finishes his motion you will see him release the handle and typically spin the frame. This helps relax your hand as it tenses up if holding the racquet too tightly for too long. Spinning the racquet is more mental. Either way the position would be continental and the reason behind is due to the next shot he will hit. Since continental is a "neutral" grip for this question Federer will have to switch to either a backhand grip or a forehand grip on the next shot. Being in the ready position with a FH grip and then suddenly switching to his BH grip would not make sense as it would simply be a mental error from Federer. Since we are conditioned with using grips and when to change it is obvious that Federer would be in continental when awaiting the ball.
2.does he change the grip from neutral when playing either forehand/backhand while under pressure, like on 1st serve returns, when there is little time to prepare,
Again you would always be in a "ready position" so this question does not make any sense. Think about court positioning. If you come in to volley and plant yourself you want to stay neutral in the middle. If you lean left (as you are suggesting in your example) and suddenly your opponent goes to your right, you are screwed as you now need additional time to adjust. The same applies with the grip when returning serve. Federer's grip varies depending on how he decides to return serve. When you have mastered grip changes then "little time to prepare" is irrelevant compared with timing and footwork.
3.he uses eastern classical grip on his backhand, so I guess it's easier for him to change grip when returning (if so), but how do guys that play a more extreme grip one handed backhand (like Henin) cope with 200kph serves, do they change the grip?
You are confusing his backhand grip with his backhand return of serve and blocking. Three different motions. The BH and BH return are the same grip but Federer uses an abridged grip when returning a BH block. This is more of a semi-eastern as you need to have a firmer grip on the bevel in order to reduce racquet vibration.
Run-down of grips
Forehand - semi-eastern. Not all FH shots are the same so he uses semi-western for specific shots.
BH - Full eastern which allows him to come from underneath the ball and rotate over it to create top spin
Serve - semi-eastern. I find it odd that Federer does not use continental for serving.
Overhead/smash - semi-eastern for most. Continental if he slices the smash
Offensive/defensive slicing - Continental for both FH and BH shots. VERY important to note.
Some of those grips are debatable but the overwhelming point is to understand the different grips for different situations/shots and why a player is doing that. Playing with incorrect grips can (over time) put someone in the hospital. I cannot tell you guys how many people I have seen hitting slices with eastern or semi-western grips
