to answer your question
1)yes it is stable, you dont need a chest strap, the way the straps are configured holds it in place and no they dont slip off. But keep in mind when you put it together dont tight'n them to much. you want to beable to get out of it underwater. the belt is what really hold it on.
as far as the LP hoses, if you wear this rg in a DIR config, Which you should IMOP, then the SPG
is on a 24 in hose clip to your left belt (ABS) d ring. Computer wrist mounted, Octo bungied around your neck also on a 24 to 26 inch hose and your primary on a 5 to 7 foot hose.
The 5 to 7 foot hose is also a DIR config and is there so that you can pass off your primary reg to an OOA diver (OOA = Out Of Air) thus giving him and yourself room to work.
If you have anymore question on Halcyon setup or DIR ask away or feel free to email me
If i don't have the answer i know the divers that do. I dive with a rather large group of DIR divers in California.