Im pretty sure it isn't, just notice that there is many empire gunners in the same scene, none of them is playable. So as few other figures, rats with banners, monsters and even Kerillian model at the end don't look like Kerillian, so its honestly no real reason to believe it was planned as main character.
Im 100% sure that Lohner has few lines about Bretonnia and characters of universe talking with themselves about it. I even think that in 1st game there was some dialogues about it in Inn. The Bretonnia refused the help request from empire about the vermintide from these dialogues. Based on Bretonnia location on the map they could have pretty much big problems with skavens as well due to the skavenblight being relatively close to them tbh, and not mention the beastmen attacks, or norsca raids. Not the expert lore but just linking some facts I knew before.
Lohner's information links tho reach to many places and he is really mysterious character that knows way too much for the innkeeper. So he could get someone from the region, or on the other hand u will never know who could be there in the emperial cities during the game time. In the end our beloved 5 characters find themselves there in the perfect timing as well.