Can confirm 300 is not needed in all 7 to scribe. I'm 300 in a few including smithing, but I scribed and combine of maul was success. Before scribe I got the Shifting Glaive
I can confirm that not all tradeskills have to be maxed. I had my Rogue buy and scribe it, he is maxed in Smithing, Pottery, Poison Making (Not required), 275 in Baking, 88 Tailoring, 0 Jewelcraft, 213 Fletching, 162 in brewing plus 200 Fishing. So as you can see, you don't need all of them maxed, probably just one.