So what are the vulnerabilities in Signal? And how is Olvid better?

At least 2 miles

I would think that you could leave a Rosetta Stone with directions on how the data is stored and read. It wouldn’t take much, I think. “These glass things contain information, here’s how it is encoded. Here’s the requirements on reading these”. You could start off simple and have a rudimentary one that can be deciphered by hand that describes how to make a device that can quickly pull information from a few others that give directions on how to build another device to read the high capacity ones. You don’t need a specific filesystem or computer to read it, you just need to know how to decipher it and that it IS data stored in a certain way, not just cool looking glass art.

No, 4kwh is about the daily amount of sun hitting 1 square meter. Solar panels can capture about 20% of that