Hello,
1. Do i need to configure asr-groups in transparent mode? What will happen if my packet (or now more accurately frame) will return to the standby context of one device, while the initial packet passed through active context on the another device (contexts are in the same group but on different physical devices)?
You only need to configure ASR groups if your routing environment would match the scenario you outlined (a return packet arrives at the unit running the Standby context).
2. In 8.4 we received new feature called BVI interfaces. How this feature integrates with failover functionality? Can we now use multiple BVI bridge groups for multiple vlans (instead of bridging a single pair of vlans in single context)?
You can configure up to 8 bridge groups per context to achieve this.
3. When implementing active/active load balancing with BVIs do we still need to use multiple context mode?
Active/Active failover is only possible in multiple context mode.
Hope that helps.
-Mike