Thomas, I always spray all over the decals. I don't like when the surface of the cab is not as glossy as the decals or vice versa. I simply don't like the decals to appear on the surface. I like when it looks like there are no decals at all. I do it with all kinds of decals, out-of-box ones, KFS ones, the ones I had printed by decalprint.de, the decals I printed on my ink-jet printer and I did so with Flemming's Ceusters decals.
It's a thing what all aircraft modellers do.
There always is a way how to do it however you have to give it a try somewhere the result is not important like a testing surface or part not for use. You need to find the proper combination of decals and lacquer. You also need to be easy on the spray or airbrush - the heavier the coat is, the higher is the risk of damaging the decals.
Now, let's talk about the lacquer. Your experience is not surprising - it really happens when the paint is too aggressive. Some decals don't mind anything, some decals are very sensivite.
The less aggressive lacquers are the acrylic ones (Model master acrylics, GSI hobby color, etc.) - you can use them on anything you want. The are fine and I haven't seen a decal which would be damaged by water based acrylic lacquer - no matter on the thickness of the coat.
What is fine for most cases are the enamel lacquers, for example the Humbrol one. I used it on my Mack Magnum and my Bedford O series. It reacted with the KFS decals slightly but there was almost no damage and the result was fine.
My latest experience is using the GSI (Gunze Sangyo) Super Clear which is lacquer-based paint (special type of acrylics) and quite aggressive. I used it over the KFS pinstriping and I had the same experience as you have - the decals were gone. Too aggressive lacquer and too heavy coat of the lacquer. However, the decals from Flemming Pedersen were untouched and had no problems with this type of lacquer.
I therefore went for automotive twin component automotive lacquer (based on the came thinner as the GSI Super clear) and the result can be seen in the pictures above - no damage to the decals and superb surface.
I hope this helps. Let me know if you need to know something else.