Nikitas,
You are right to say that expulsion, population transfer etc are contrary to international law, human rights conventions, etc. Unfortunately, perhaps, these laws and conventions did not exist until towards the middle of the twentieth century onwards. In fact the basis of modern international order, including the basis of states today, is based I would argue on those massive population transfers - massive human brutalities. Some of these brutalities were 'legal' in the sense that state leaders and negotiators agreed with each other that XY & Z would be transferred/exchanged for AB & C and that this should be done through a formal agreement. Many transfers/exchanges were also done 'legally' when carried out 'domestically' for example, under the Nazi Third Reich across the Europe under its rule. And of course many were done without a care for 'legality'.
To be honest I don't understand your argument regarding 'signing away' or non-forthcoming signature'. It seems to me that once a law has been instituted internationally and so long as that law is in place there is no possibility that rights can be signed away. After all if a right is established as inalienable then, by definition, no one can alienate it either for themselves or on behalf of someone else.