Re: Secret behind Obito's Kamui?
Here's the thing though. It is unclear whether Shisui knew that Itachi had being ordered to massacre the clan or not. Personally, I read Itachi's memories(what he showed Sasuke) chronologically and in doing so, I don't think Shisui knew of the massacre. Shisui gave his eye to Itachi, then Itachi went to the meeting with Danzou and the Elders, then he encountered Tobi.
Furthermore, Itachi and Tobi's meeting was so brief and SPECIFIC that imo, it had to have happened after Danzou had given the massacre orders. Itachi tells Tobi that he would let him get revenge on the Uchiha in exchange for sparring Sasuke and Konoha. It strikes me that, that specific detail is the key here because up until then, Shisui and Itachi were working behind the scenes to SAVE their clan, not massacre them.