My heart goes out to you since the situation you've described is so terrible. Without repeating what you've already said, I can only remark that your research into the car's past has left me curious about the care and driving habits of its prior owner(s).
In the end, a KIA dealership inspection is required before a decision on the overall cost of engine rebuilding (or replacing the engine entirely) can be reached. It will be costly anyway, but KIA could consider providing financial aid, though they are under no obligation to do so.
The risk of going to unofficial, less expensive garages rather than KIA dealerships has been brought up before forum members previously. I'm going to assume that KIA does not recognise the validity of service book stamps as proof that maintenance has been carried out according to their standards, given what you have said. If it turns out that the automobile has been serviced at a KIA dealer, then KIA UK needs to step in and take responsibility, working with their designated dealer, to resolve the issue.
However, between maintenance, it is still the responsibility of the owner or driver to monitor the oil and fluid levels. As many of you who are frequent readers of this site are probably aware, there are some owners who, in between servicing, never even check beneath the hood, save to fill up the windscreen washer fluid tank.
I hope everything works out well for you in the end.