All great questions; I suggest you start with the README found here:
Almost all of your questions are answered in the README document, including detailed feature lists, step-by-step installation instructions, screen shots and tips & tricks for using the SmartApps.
Then follow the activity here:
As to your specific questions?
Yes, it is stable, but under continuous improvement. You can stop anywhere, and choose to get updates as you wish.
Yes, you can use the Humidity Alert SmartApp to control your dehumidifier based on the thermostat’s humidistat. That app’s existence influenced me NOT to make another redundant Helper for that functionality.
Yes, the Suite contains a Helper SmartApp that will turn on/off switches/dimmers based on Operating State changes in the thermostat(s). And a whole lot more…
If you use the SmartThings IDE integration with Github, it’s really easy to keep up to date when updates are released.
You can also use the Community Installer to maintain the latest version: