So then the other crime was only a crime because it was done with the intent of theft? Yes.
Does the "other crime" relate to the threats? Yes. I assume it isn't the threats themselves, as that IS usually a crime in and of itself, and also because it was already mentioned in the puzzle statement. Correct. But maybe it enabled the threats? NO!
Otherwise, did the other crime enable the thieves to know when the paintings had been removed from the galleries? No, but OTRT. allow them to know what car would be transporting them? No. You say that it did not involve touching the paintings, but this was OTRT... did they touch something on which the paintings were mounted? No. maybe affixing a GPS transmitter to them somehow so that they could be tracked? No.
Is one of the above FAs that the letters were threats? No. Were they warnings? No. Were they sent by the criminals? Yes.
Did the other thing done in the gallery assure that the paintings would be moved? YES! Assure the manner in which they would be moved? Yes, if I understand the question right. Did it involves the letters received? Yes.
A couple of private art galleries receive letters threatening them about planned thefts of specified, priceless pieces of arts within certain time. All galleries have modern, well - functionning security systems and also well - trained security guards. After having received the letters, all galleries even strenghten their security efforts. All galleries function normally, undisturbed, nothing out of ordinary happens. Some time after the dates specified in letters, the galleries become suspicious about said pieces of arts. All pieces of arts specified in the letters are proven forged.
We already established that the paintings are stolen from the cars during transportation. Criminals damage the cars in the way they get broken in a definite part of their route. Then criminals appear as towers and, while pretending to be looking after the damaged cars, switch original paintings for the copies.
The criminals also do something else that involves the paintings and is connected with the letters. This activity assures the criminals that the paintings would be moved and also he manner in which they would be moved.