If risk assesment is carried out before BIA and before business continuity strategy, when are unacceptable risk mitigated? should they be mitigated/avoided right after identification and evaluation or after the BIA, or during the business continuity strategy?
First lets separate the moment for the treatment choice decision from the moment for its implementation.
The decision to mitigate/avoid a risk should be made after the risk evaluation, considering the criteria you defined in your risk assessment methodology.
Regarding when and how to implement the mitigation/avoidance for an unacceptable risk, you should consider the results of the BIA to decide.
In the Business continuity strategy (or in the Implementation plan which will be an appendix to the Strategy) you should define when an how the mitigation/avoidance will be made - the higher the risk, the sooner you should try to mitigate it.