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.