The last stakeholder can be an attacker who produces many valid blocks, but it isn't much different than attempting DoS on the Bitcoin network, it's discussed in section 5.1 of the PoA paper.
What if the last stakeholder will send lots of
valid blocks. Like he will connect to all peers and send them different, but valid blocks (the "cost" of this kind of action is comparatively low).
How they can choose the "right" one?