Yes, the code all runs client-side so your data never leaves your machine. Feel free to check the source code if you don't believe me or even download the files and run it yourself.

Go to Google Takeout, make sure the Hangouts option is selected and create the archive. When it is ready open the zip and extract all files. In the Hangouts folder there should be the hangouts.json file you're looking for.

A basic text editor like notepad should be able to open any of the files. Even many browsers can open them.

Maybe. I'm only doing this in my free time as a side project but I'm very happy to get feedback. Submit your request here and I'll see what I can do.

To reduce file size, the CSS styles in the HTML format are shortened to abbreviations. They are: m for messages, s for sent messages, r for received messages, d for message details (sender and time), c for the circle next to the sender's name and nl for the new line that forces messages to appear below the last sent message.