The sensor has a little double sided tape attached to it. Just peel off the cover paper and it should be held in there ok. I'm only using the zip tie and it works fine for me; I'm using it on a mountain bike. Just make sure you tighten it real nice.
I was confused about the extra stuff coming off the sensor mounts ... cut it off, that's what I've been told to do, and that's what I did.
The problem with the Astrale is that the sensor mounts are weak. The cadence mount is particularly vulnerable to breakage if not handled carefully. Mine broke twice, and what I did instead of repairing it is that I used two larger zip ties to hold it in place ... one to secure it to the chainstay, the other to space the sensor away from the chainstay and closer to the crank. It kinda looks like a braid.
You can get a new beefier sensor set, but they cost about the same as the entire unit, so don't bother.