I will get the quick stuff out of the way: buy the key...it really is worth it; I found myself making multiple files with different changes to each one, and it helped a bunch on the faiz unfolds. Now, I have to keep the program open all the time, or risk losing all my work.
I took a look at decade, and amazingly you could build it without the bars on the helmet, but you would have to be precise with the bars when you add them on later. The ones that would seem easier to add things to later would most likely be both decade and diend; the other helmets need to be built as one complete item.
The double helmet I will try to finish and upload hopefully by this weekend, or sometime early next week(remember, you will have to unfold as I can't). I'm trying to figure out how to join the helmet back together for faiz so it still looks as one uniform piece, since all rider helmets are a standard two-piece format(looking at ebay helmets/watching old episodes is a good example); unless your head is small enough that you can put on/remove the helmet without any problems, then it needs to be split down the sides.