No. You can only include 0-1 Herdstone per army and, whilst an allied contingent may be in effect a small army within a larger army, in this regard you are ultimately playing with but one army.

However, if two players partnered for a doubles game (as defined in the Matched Play Guide), they could each take a Herdstone.

In cases such as this, both outcomes can apply. If a natural double is rolled, the spell is cast as a perfect invocation and the caster suffers a hit from the Plague Chalice. However, the caster and any other members of their coven will then also suffer the effects of a roll on the Miscast table, as per the Dark Coven special rule.

No.