Yekke, your last post is obviously the basic answer, especially since we can say that in Chutz La’aretz he didn’t do Kedushin, as the Ramban writes regarding all other Mitzvos.
I am not sure if you understood what i wrote fully; I don’t think i explained myself correctly.
Kiddushin is a Chiddush Hatorah – i don’t mean they didn’t KEEP that law, i mean that Kiddushin before Har Sinai doesn’t work (The Rambam says that before Matan Torah there was Nissuin only and no Kiddushin; Kiddushin can only happen after Matan Torah) – therefore the Avos were not going against the Torah at all by not doing Kiddushin