There are way how control or change variable in WidgetBlueprint through WidgetComponent from c++?
To interact with blueprint classes from cpp you can use UE reflection system:
for example
UObjectProperty* LocalPlayerOwnerProp = FindField<UObjectProperty>(NewActor->GetClass(), TEXT("YourPropertyName"));
allows you to find a reference to a field declared inside your class (even BP)
LocalPlayerOwnerProp->SetObjectPropertyValue(Value, ObjectInstancePointer);
this code assigns the object containing in Value to the property represented by an LocalPlayerOwnerProp inside the ObjectInstancePointer( your blueprint class instance )
UObject* SomeObject = LocalPlayerOwnerProp->ContainerPtrToValuePtr<UObject*>(ObjectInstancePointer)
allow you to read the value from an object - this converts pointer to an object instance to the pointer to a property
If you have to set simple struct or plain integer\floats you can use just ContainerPtrToValuePtr for reading and writting.
To find more samples just run searching of FindField thru the engine cpp code