This question seems to have generated a lot of heat but not so much illumination. As IA Petr Harasimovic suggests in one of his comments I think I know the answer although it is nowhere near as simple as he suggests.
Let me start by quoting from the preface to the FIDE Laws of Chess -
The Laws of Chess cannot cover all possible situations that may arise
during a game, nor can they regulate all administrative questions.
Where cases are not precisely regulated by an Article of the Laws, it
should be possible to reach a correct decision by studying analogous
situations which are regulated in the Laws. The Laws assume that
arbiters have the necessary competence, sound judgement and absolute
objectivity. Too detailed a rule might deprive the arbiter of his
freedom of judgement and thus prevent him from finding a solution to a
problem dictated by fairness, logic and special factors. FIDE appeals
to all chess players and federations to accept this view.
This seems to me to be one of those cases and I originally asked the question to find out what others thought.
Let's start with the question of the "illegal move".
Suppose player A while making his move and pressing the clock knocks over some pieces. His opponent, player B, immediately presses the clock back so that A can restore the position on his own time. In view of 7.8.1 is this an illegal move?
After some thought the answer is obviously "No". 7.8.1 applies when it is B's turn and the only correct possibilities for B are to resign, claim a draw or make a move. In this situation the position on the board is not the position in the game and so it would be very wrong for B to make a move and press the clock. If he cannot make a move then it cannot be wrong for him to press the clock although it may be undesirable.
Player B has two legitimate courses of action. One is to press the pause button and call the arbiter and the other is to press his clock to allow A to correct the position on his time.
Which one is the better option depends on the situation. There are problems with both courses of action.
If B just presses the clock then when A has put the pieces back he will also press the clock and both players will have had an additional increment to which neither was entitled. In standard play with say, 90+30 (90 minutes for the moves with a 30 second increment), this is much more serious than in say, 3+2.
If B presses the pause button and calls the arbiter then he will give him an additional 2 minutes (1 minute in blitz). Whether this is good or bad depends on the situation.
In a 3+2 game the problem with this approach is that it will take time for the arbiter to come, it will take more time to explain the situation to the arbiter and yet more time while the arbiter adjusts the clocks. This additional thinking time (for that is what it is) is very large compared to the increment and probably to the total thinking time left for the players. This will very likely advantage one player much more than the other. This advantage could decide the game but in any case is likely to be much greater than the additional 2 second increment that each player gains if B just presses the clock.
As an arbiter what would I like you to do if this happens in your game when I am the arbiter?
In a standard game with something like 90+30 please press the pause button and call me. In a normal situation I will adjust the clocks and you can continue once the position is restored.
In an extreme situation, say you have 30 minutes left and your opponent is playing on increment I will tell you to play on without adjusting your clock since this extra thinking time would immediately advantage your opponent. Instead I will come back in 20 minutes time and if the game is still in progress I will give you your additional 2 minutes then when you are about to need it.
In a rapid competition I would give the same advice / request.
In a 3+2 blitz or similar I would suggest you just press the clock. In such a situation I think this gives the "best" results. There is the least disturbance to the players, the additional 2 second increment is tiny compared to the additional thinking time available if the arbiter is called and so is much less likely to change the course of the game in an unfair way.