Is the action of the Imam, in interrupting the recitation for supplication and then resuming it, considered valid in Tarawih or Qiyam prayer, and is the prayer invalidated if his action is not valid?

It is desirable for the worshipper, when passing by a verse of mercy, to ask [of Allah], or by a verse of punishment, to seek refuge [in Allah]. This applies only to supererogatory prayers for both the Imam and the congregant, and it is not desirable in obligatory prayers. The Imam should be brief in this. As for interrupting the recitation for supplication and the congregants saying "Ameen," this is an innovation (muhdath) and not legislated, and it must be avoided.