I know nothing about hardware versions of OpenSprinkler, but I have a couple thoughts about the rest of your post.
In my experience, hardware like OS isn’t going to be bothered by a few hours in a below freezing environment. Here, outside of Phoenix, AZ, below-freezing temps are common during the Winter. This year, we had a couple nights of 23 deg F. I have two small Orbit controllers that live in a metal box fastened to the North side of my house. The controllers have been through several winters and have never missed a beat. With the power dissipation of the transformer and the electronics itself, and the very few hours that is spent at the lowest temp, the controllers are probably not even remotely close to being stressed.
Summer, on the other hand, can bring conditions that can be troublesome. If there is direct sun on the enclosure for many hours, little wind and very low dew point, the temperature inside the box may become uncomfortable for the electronics. In my situation, I put some 1/2 foam inside the box to minimize the temperature rise for the short time that the sun hits the controller enclosure. During the really hot part of the summer, and especially if we are traveling, I put another object in front of the enclosure to shield it from any direct sunlight. I’ve never had a problem.
An alternate to Wi-Fi that works very well is to install a powerline carrier Ethernet adapter along with the controller inside the enclosure. If you don’t want to leave the adapter running, you can use an remote control power switch (radio control type) or an X-10 appliance module, to turn the network adapter on when you want to use it.