>Which country could it be? Magadha?
सायण's interpretation of उपा॑रण isn't intended to be one specific country, but basically any foreign land hostile to Vedic rituals. At one point that would've included मग॑ध, yes. There would be some ब्राह्मण॑s living in such areas, who gradually spread Vedic civilization over the centuries.
>where brahmin rituals are not followed, this verse wants Indra to not take those sacrifices
According to सायण, yes. But his interpretation doesn't really make sense given the context and the original expansionist mindset of the Vedic religion.