If the furniture didn't leave permanent marks when the carpet was dry, they won't be permanent now. Dining chairs shouldn't be a problem, unless they're really heavy.
As said above, heavy stuff like big tables and 3pc suites etc should go back exactly where they came from.
Don't forget that indentations are a 3-layered sandwich. You've got the crushed pile, then below it the backing will be stretched and finally under that the underlay will be crushed. Temporary indents from light furniture on a 100% wool carpet will look quite severe, as you've bulked the pile of the carpet back out when you cleaned. The difference will be more pronounced.
One word of warning about ice cubes... I'd be very wary, either of of doing it or advising your customer to do it.
Quick quiz:
Wool + wet for a long time = ?
Ice is made of water. The temperature doesn't do anything, therefore just dampening the area in a controlled manner and leaving it to dry will do the same thing. Personally if I had to take any remedial action at all I'd set up my machine with a hand tool and simply extract over the depressions, making sure the carpet isn't left any wetter than after a normal extraction clean.