Update test case from defect

Well, you can log events using comments and reassignments within a state are fine. It looks like you might need an augmented work flow for defects. The built in assumption is that a work item is implemented by a single individual though it may be verified by another. The expected mechanism is that the defect will have child work items for the various kinds of rework needed; i.e. code, documentation, test cases, requirements, etc.
Your design might work, but looks a little complicated. A list of testers would require a Team area with members, which could cause other issues. I know of no mechanism for a selection list filtered by role. Similarly notifications are very limited in RTC.

  Thanks Paul, i finally go with simpler approach: