Yeah, I expect sneakernet to work pretty well. A few weeks is way better than nothing. Also, you don't need to use an active internet connection to make reliable zero-confirmation transactions (see lightning network, although it's true that monitoring for confirmations requires internet data- so you have to set the timeouts to be way way into the future, which is reasonable and not unexpected).
Yep. Double-spending zero-conf transactions can work as long as you are using a payment channel (where the other side is receiving each transaction). That's how the "payment channel" concepts work.
