This should be no different to using your management/communication skills to avoid having to deal with a bigger issue later. For example if I see a defender running towards a free kick location to stop a QFK I shout "move away". If the keeper is holding the ball close to 6 seconds or a player is taking his time for a restart I shout "let's go". Or if I see a defender being too aggressive in a challenge and is in danger of giving a FK or penalty away, I shout "be careful".
In the case of a back-pass I don't say anything for an obvious back-pass, the keeper (and everyone else) should know. In a not so obvious back-pass it helps to say "no hands". If you get a complaint from the opponents, just quietly tell them you are not sure if you would have given a free kick, you just didn't want to have to make a controversial decision one way or other and you do it for both sides.
The added bonus is, if you do have to give a free kick or sanction, it is much easier to sell the decision.
Yes, but be very careful what you say. I once tried to help a keeper by shouting "Don't use your hands", only to see the keeper pick the ball up and then try to claim that he hadn't heard the word "Don't".
I have heard me saying "passback" if I deem it to be so, and this has been standard for certainly officials I have worled with over the years. Granted this comes from regularity of using comms but its a habit I am still in
If its one of those where the ball is trundling towards the keeper and I take an educated guess about what might happen next, I shout. " keeper, you can lift"
As alluded to above, thats just managment of the game and no diff to proactive things like, hands down, no pushing, dont foul, let the keeper kick, and so on
This past Saturday, for the first time in a while, I had to penalise a clear pass back. The goalkeeper argued that he was entitled to pick it up as the pass had been volleyed rather than played along the ground!