How to structure a Multi-period Institutional IPS ?

If you have an institution that have multi-period investment horizon. Each period has totally different objectives, strategy ,target assets classes, and assets allocation.

Shall we focus on the second period which is the ultimate or detail the two period strategy and then give details on the transition?

I believe that is why we have multi period structures - thus two period strategy.

You are overthinking this. The simplest way is to construct an IPS for period 1, manage to it. When period 1 is over, meet with the board and cosntruct IPS for period 2…and so on.