Yes, Sqlite and Postgres are supported since the version 1.2.0.
Miniflux uses an Ajax request to refresh each subscription. By default, there is only 5 feeds updated in parallel.
Probably, but your life is too cluttered.
Miniflux is a minimalist software. Less is more.
Report the bug to the issues tracker.
You can report feeds that doesn't works properly too.
Miniflux is tested with the latest versions of Mozilla Firefox, Google Chrome and Safari.
You can find your API token by going to https://api.pinboard.in/v1/user/api_token/.
The Pinboard token should be formatted like that: bobsmith:12FC235692DF53DD1
.