clash detection

i used that one to solve the problem but it is not the default one , how I can make it default

But that is not a viable solution as a mitered cut is proper for unification and centerlines.

I don't understand.... the two walls in the image already have a Bisector connection.  By using the Connect tool after the fact, you are just insuring that the connection is valid.   What I am not sure of though, is why the connection would not already been in this state.

because of the comment earlier I am wanting to make sure we are not having to replace the miter connection with the L connections as they do different things when rendered

Correct, Bisector refers specifically to a mitered connection while Parallel is a typical "L" connection.  

Though I'm still unsure why the original Bisector connection was not valid - it seems that was the cause of the clash, as though there was a true overlap and not purely coincident faces (which the suppression rules would cover).