.... and in any case even if you do not choose to preview the new values it should still export them ok - if it does not, that may be a bug [edit you replied whilst I typed this ]

the base unit of a vector editor like xara is the object not the layer - that is all the objects are independent of each other - whilst it may not impossible to do [I don't actually know], applying attributes to objects that are not selected, globally, is not the way the program is designed to work, so a layer transparency is unlikely I think...