1. I recommend using the official plugin since Steam4C2 has been completely abandoned by the developers (or am I wrong about this 
MadSpy AJ2DI?). It's not a scam by any means but don't expect updates anytime soon for that plugin. Same could be said about the official plugin since that one heavily relies on 3rd party code for better or worse and Construct's focus has mainly, but not exclusively been on mobile games so far.
Read the manual for the plugin you choose. In short, specific versions of NWjs require specific files.
2. I'd always recommend going with Construct 3 if you can afford it. Tons of improvements, both for the editor and games made with it.
3. File format is up to you, pick what you prefer to work with. There might be some file format "elitists" telling you otherwise but in my opinion none of that matters, unless you save huge amounts of data.
There are 3rd party encryption plugins out there to keep your save-data safe.
4. It's common for software or games to save inside folders with possible write access (e.g. appdata folder). I'd personally recommend a multi-folder approach. Try saving to your game's folder first, pick another if it fails. Add a timestamp or similar to always load the most recent file.
Construct 3 has conditions to check if writing to file has failed but in Construct 2 you would have to do something similar to this example:
5. Steam supports private beta branches for testing. You can even give access to friends or credible game journalists.