I had no problems with mice (I kept food in my tent with me), but I also heard the stories. It's not that big a deal to bring the ursack, so I might if I were you. There are no real places to hang anything, so you don't really have any good options other than a rodent proof bag.
Yes, you want a real windshirt with a bit of DWR. It doesn't really RAIN per se, it's just awfully drizzly, which can easily turn to sleet. I used my marmot precip ALL the time and never regretted it, but this was before I discovered wind shirts. I have the boreas as well and I don't think it's enough... You absolutely will want a hood. I also had rain pants and was so very, very happy I had them.
I used a montbell apline light parka - down - and it was perfect. I never had any issue keeping down dry, since it only came out once we were in camp. The cold really came from the wind, so if you can block that well you can get away with not much insulation while actually hiking. Most of the campgrounds are quite sheltered, and all have cooking shelters, so that helps quite a bit.
Not sure why you would want both a fleece and a puffy...but I would recommend something like a cap4 hoody or something as a sleep layer (that you could use as well if the weather turned nasty on you during your hiking) but not sure you need another dedicated insulting layer.
We stayed at the Singing Lamb hostel in puerto Natales and LOVED it! The lady who runs it was so nice, it's just a block or two off the Main Street so it's quiet and great sleeping. Bathrooms are clean, showers are hot and strong, and she cooks a mean breakfast. The place is also just two blocks from Erratic Rock, which is where you WILL go for the daily 3 o'clock talk.
I did the circuit in feb of last year and had an absolute blast. My trip report is at backpackerPT.com, and feel free to send me an email or PM if you have any other questions. jenmitol AT gmail DOT com
Have fun planning!!