Relationship with the Xenquans
It really depends on the feelings that your "Swiss" had towards the Xenquans:
- was it a colonial relationship as we have seen in the first part of the 20th Century: some of the indigenous participate in the colonial system, but the majority are not much more than slaves?
- was it a somehow protective relationship, where the colonees are feeling relatively well in general?
- was it a purely commercial relationhsip, where the "Swiss" have a relative feeling of political independence?
In the last two cases, it is likely that the political structure will continue, with a stronger independence from the Xenquans, or even a reverse relationship will establish itself.
Weaken Colonial Powers in Modern Times
I will now concentrate on the first case from before. Seeing a weaken Xenquans, it is likely that members of the overall populations, bitter from the difficulties with the colonial power, seize the opportunity to get rid of them completely. Some members of the colonial structure will also jump in to try to seize the power for themselves.
Of course, due to the effect of the virus, the Xenquans can't oppose anything. So it would not turn into some form of independence war as was seen, e.g. in Vietnam or Algeria. But most probably there will be three groups to oppose:
- those close to the power circles of the colonial administration, who want to seize the power for themselves, but otherwise leave the global organisation unchanged.
- those not close to that power circles, who want to change the system entirely (revolutionary circles)
- those who don't want to throw down the complete system, but want to make a transition to a better human/economical/etc. situation.
and a large majority of people just wanting to live their lifes and get some food every day.
Then again, it really depends on the relative power of each groups and the culture of the country. Let us consider the following possibilities.
- 1 >> 2: possibly by being linked to the army, the people in power during the colonial administration keep it, and prevent any attempt at organizing a revolution. This would probably lead to some form of dictatorship as the power is controlled by the army force. A number of African regimes, or possibly Argentina or Chile's dictatorships come to mind. It does not have to be extremely harsh, it might be "just" extremely corrupt.
- 2 >> 1: there, you get a proletarian revolution. The population arms itself, parts of the military/police join them. The people of the first group, flee or are arrested and often executed. Examples stem from the already cited Vietnam, or Algeria, but as well as the Russian and French revolutions. The outcomes may vary as much as the revolutions presented here. It can eventually lead to new Emperor (even if somewhat sympathetic of the "base" population) or to a proletariat dictatorship. But it can be a bloody business.
- 1 $\sim$ 2 you might get into a civil war. And that could be a very bloody business. It can lead to the total destruction of the political and law enforcement systems, economy, and a high number of victims. What comes later is hard to predict. If it goes on badly enough, you might see some form of tribal groups appearing. Where locally people unite, with as many forms of political organisation as there are tribes.
Case of early medieval world
I just wrote the previous part forgetting you have a medieval setting. And thought it could just stay in as illustration of how modern political ideas would be.
Considering that your "Swiss" are in an early medieval tech and culture (?), back then the military power was the almost direct source of political power. There are many possiblities, but I will explain the most likely ones.
The "Swiss" have a King or some form of leader, who was dependent on the Xenquans power.
- He was responsible for the inequality of the relationship with their overlord. And a member of his family profit from the weakness of his relative support to make an attempt for the throne. The perceived weakness and surprise makes the plot successful. The new leader gets crown, and make sure the protests against the coup get somewhat limited.
- He was born in that system but was himself unhappy at having a liege lord above him. When the Xenquans disappear he takes a firmer control of the power and establishes himself as independent ruler.
Either way, when their positions get secured they get on reforming the economy (which might entail trying to conquer parts of the Xenquans lands).
Note on Post-Apo. Due to the fact that they aren't affected by the virus, there does not necessarily occur an apocalyptic desaster throughout their land. So the political system is likely to be some transition from the previous situation.