Yeah, it would be extremely tough for them to hold onto Nivemus lands. The Omsk Bay kind of makes that a very difficult task.
Personally, I think OI should give Perdan Bastad Island and put militia down in Chaos Stronghold. Consolidate their territory. Then think about moving more onto the mainland.
Before we took a hold on the mainland we discussed this, both in realm and with Perdan, it is something that both Perdan was not interested in and something we couldn't readily give up as it doesn't change how far new regions are from Kalmar.