I have installed really nice Belchertown Skin a few weeks ago and had problems with weatherRangeChart too. I solved my problems last weekend, so i hopefully can help you!
Have a look at Belchertown Chart Wiki at
https://github.com/poblabs/weewx-belchertown/wiki/Belchertown-Charts-Documentation. Don‘t use [[[outTemp]]] for the observation, but [[[weatherRange]]] and set observation with keyword range_type = outTemp below there. For your example:
[[weatherRangeChart]]
title = Temperature Ranges for this Year
show_button = false
time_length = year
css_class = "weatherRadial"
[[[weatherRange]]]
range_type = outTemp
polar = false
Important - this was my problem - weatherRange Chart doesn‘t work with Belchertown Skin 1.0.1, so I have to upgrade to skin version 1.1.b8 development branch.
Your question 2 - I am not really shure, if charts on the homepage can use full width.
Hope I can help a little bit.
Dietmar