I can only answer the first question. Yes you can clearcoat on decals, especially those that you want to make it look as though it is painted on for example rally car liveries. But make sure the first 2 to 3 coats are very light mist coats or else the decals will 'melt'. Also make sure the decal is fully dry (let it dry a few days) and totally sticking on the plastic. Should it has an edge that is not sticking to the plastic, chances are that edge will be 'eaten up' be the clearcoat.
I have used Gunze and Tamiya clearcoats on decals with not much problems.