1 - openresty is nginx and many official and unofficial modules. From what I've asked the experts, there is supposed to be no difference in the source; openresty is just a single package of many modules with an easy to use ./config and make process.

2 - The nginx module and lua lib ecosystem seems to have most of the basics in order to roll a high level app framework, but only a half baked one exists thus far: https://github.com/appwilldev/moochine Also: https://github.com/antono/valum https://github.com/pintsized/lua-resty-rack

My impression is if you have the time to roll your own sinatra-like framework, it should be pretty straightforward.