- can I develop custom UI panels/windows using my own controls(buttons/sliders/etc) ?
Yes. You are writing a DLL so all the Windows API are available to you - which includes GUI objects. If you're talking about Adobe widgets then no, you have to stay within their framework (eg filters, exporters)
- can I get all info about current project(imported files/timeline/effects/etc) ?
Partially. You can get timeline and clip information but it doesn't tell you about effects that are applied.
Download the SDK and look at the PDF document. Also from that look at the API header files it references - they have comments in them that expand on the document information (especially for things like widgets and what information is available about the project)
