Yeah, but I dont know how to assemble a cross layout from images, however if you already have a cross image you can ofcource use it, 2) is true, seams will show if you use jpg as source, or DXT compression for the cubemap.
Also if you want to make an ingame envmap for metal shaders, you can use this methiod:
http://www.leadwerks.com/werkspace/topic/11224-skybox-rendering/#entry82736