Sorry for the delay.
1. Can you use Biml to create SSRS reports?
Not at present and I understand it is not currently on the roadmap
2. What version of .NET is BimlScript using?
3. Can you kick off package generation from Biml through PowerShell?
Yes and this is a Mist only feature.
4. Can you convert an existing SSIS package into Biml? (or do I need Mist to be able to do that?)
Yes and again this is a Mist only feature.
5. What are the capabilities of Mist over just using SSDT?
Here are a couple items: Analysis Services support (Mist 4.0 will now support Tabular), Transformers, Build Automation, SSIS Package Import, Full IntelliSense.
Hope this helps.