1. No problem doing that as long as you don't try it with the magic powders W296 and H110.
2. This is one of those "I sure wouldn't do it" things. Will the cartridges blow up on the first shot? Probably not, maybe they never will. It all depends on the shells you bought. But, .38 Spcl +P shells are a little more substantial, especially in the web area, so why not just buy some of them if you want +P loads? Or buy a box or 2 of +P cartridges, fire them, and use those shells for your +P loads. It would be a whole lot cheaper than sending your damaged gun to a gunsmith or sending your damaged bod to the emergency room, wouldn't it?