"ElAl serves Kosher" means you will not get milk after a non-dairy meal from the FAs in the isles. It does not mean they will not serve you coffee with milk in the galley if you go there alone and ask. The meals are Kosher, the plates an utensils are separated and kept Kosher but it does not mean anyone on board MUST keep kosher through the flight. You just don't get many opportunities not to ...

I believe one can bring anything he/she wants on board and use it as much as they want to. I don't believe one can be sues for eating a cheeseburger on a flight. However, I cannot predict the behaviour of the surrounding passengers at a specific flight.