Indeed the flows are a natural evolution of the actions on your page and allow you do to much more complex logic. Action trigger just a single action and have no return value indeed, while flows can contain more complex logic, has a bit the same power as server connect actions on the server but flows are all on the client side.
Inline flows are indeed just inline per element, while page are on the page for reuse on that page and app flows are more global indeed to be used on multiple pages.
Inline and Page Flows indeed have access to the page components and can run their actions, while App Flows because they are global or just global actions on their own that have no knowledge about the page that runs them.
App Flows are global and some components are dependent on the frameworks used on the main page or capacitor/cordova. Like SQLite plugin requires capacitor. But indeed as it is selected as runtime, we should be able to detect it and allow the SQLite plugin usage in app Flows. So will see if we can improve this.
Well all actions in a flow are sequential. They are action promise based. So always run one after another and only after the previous was successful. If you do run an external app flow, then it is waited till it completes to go to the next action steps.
Keep reporting any bugs as separate topics, but do make that you list the exact steps to reproduce, so we can easily track them.