From a lore perspective this would be a heavy deviation of Retail WoW, but i don't imagine that as a bad thing, all that's really needed is a disgruntled group of GIlneans to want to not be under the control of Genn anymore and as luck have it there was already such a group.
Darius Crowley started the North Gate rebellion even prior to the Worgen curse running rampant in Gilneas, he has multiple reasons to dislike Genn and challenge his orders.
- Crowley was ignored when he insisted Gilneas should send a meaningful force in the second war.
- He then was ignored when he pleaded Genn to remain in the original Alliance.
- The small force ont he third war was met with the displeasure of Genn.
- When the wall was built, his lands were left out of protection and he wasn't compensated for it.
- He was then arrested due to the rebellion and only set free being ordered to go fight worgens.
- Crowley might not even be aware that the worgens he was killing were once fellow gilneans.
Following through to the part where the night elves help the citizens to control their curse, where Genn reveals he's been a worgen all along and was getting helped by Belysra to keep it under control.
So many people would have a problem with this.
The king has been lying for months about his curse, he's been getting help from an outsider after preaching isolationism and more important than anything he knew there was a way to contain the curse all along and kept it solely to himself while still ordering the execution of cursed Gilneans that maybe could've been saved.
Crowley can just lay it all on Genn and decide to leave the kingdom then and there, he can eventually contact Jaina seeking shelter for him and the refugees that decided to leave Gilneas, she can be unsure what to do but she also literally fought alongside the new Horde, she knows Thrall and Cairne. It might not be the most pleasant option to Darius, but what else can he do at this point? March his survivors to Lordareon and be buddies with Sylvanas?
Being transported by ships or magic, where would we put these refugees? Who would be able to help them control the beast within? Clearly the Tauren, initiate them on the path of the great hunt, help them find balance using their shamanistic and druidic magic same as the night elves did.
Crowley leading the refugees that yearn for freedom and are seen as "monsters" fit the horde so well, they're not burdened by kings and nobility so much as they're held on their honor.
And even if Crowley only sees this arrangement as temporary and intend to one day return to Gilneas, that suits the Horde just fine as well.