The regulations regarding inserts have been out longer than 2 yrs, the problem was when they brought them out no one sold them and like everything oftec does it is never very well publicised so no one fitted them. The merchants should not be advising trained engineers or even the public because they are not trained in the regulations. If they advised something that was wrong they could be sued in a civil court.
Oil regulations are not backward enforcible you only have to look at the number of oil tanks that dont comply with current regulations.
Yes compression fittings will eventually leak on soft copper, you will find it wont be a drip but only what we call sweating which is a damp stain. The soft copper pipe under compression by the olive eventually gives up and is compressed to the point that it does not push back against the olive, the use of the insert is to make sure the joint is compressed by the olive onto the insert which will not give and therefore wont weep.
The use of the inserts as I understand it is a british standard, how many merchants know about british standards. It sounds like you have done the best you can and I am sure there are a lot of people out there even qualified oftec engineers by the sound of it who would have done exactly what you have done, I would not be happy if my own oil installation was not done by the book whether I agreed with it or not. All oftec engineers stand to lose their registration if they do not do their job properly and we do get inspections.