i think nikila may have told shiv at the last minute on the wedding day, in order to persuade him 2 marry gauri.
if the kundali plot is correct then maybe nikila told him abt his life in danger and gauri is the saviour in this whole thing.
for his mums sake he did all this, from there he must have found out his mum planned this and machan and shanks were a part in this.
i think that shiv thinks he is a 'PART' in this plan, because he blames HIS kundali and that if the kundali situation didnt come up then maybe this wouldnt have happened in the first place.
judging the scenes on the wedding day, it did not look like shiv knew the plan from the start.
but i think nikila told him abt her plan the moment mahen walked out of the room after threatiening shiv 2marry gauri otherwise he and palk wont get the property.