Cannot find Nameless Seer with dedicated Atlas, Scarabs & Packsize-Maps to find him.

If nothing else, at least it's a mist farming strat

Exarch/Eater influence doesnt nerf the seer spawn. It just has a high chance of having the influenced packs be the selected monster type that you must kill. Since those packs hide behind a proximity seed with a small hit box, there are often unspawned packs left when you finish map, you can see this by looking at the monsters remaining counter. If you full clear the map down to 0 remaining, influence should have no effect on spawn.