'I came not to put away the law but to fulfill it.'
Ie what was defined before as sin is still sin. Strike one. Christ's death and resurection did not change this.
OT worship of the dead was done *knowing* it was explicitly to ask of someone other than God something which was not God's will.
Couple of things here, 1) we weren't talking about worshipping the dead, we were talking about communicating with the dead. 2) No they didn't know it explicitly and that in part is the error of it. Saul thought he was talking to the prophet though he spoke with a demon. God put him to death for trying to commune with the prophet.
Christian prayers with the dead help to conform your prayers to someone whose faith in Christ has been perfected, explicitly because it helps conform our wills to Christ's, and perfect our faith.
Who cares. This is like saying that a gun is a beautiful thing and attempts to be as accurate, etc.. it doesn't make murder any less a sin. You can talk about conforming wills to Christ, but praying to a dead person who cannot hear you or respond to you and is in itself a sin doesn't conform your will to Christ, it rather repels one from Christ. Or did you not know that is the nature of sin. Sin is rebellion against Christ. A rebellious act is not one of conformity. You are trying to define black as white and white as black.
Communicating with the dead is impossible and the attempt, as it happens, is a sin. Pure and simple. If you'd done it as pre-christ Jews, you'd have been stoned to death for it. Now the proper thing to do is warn you because the penalty is reserved by God now and is owned by Christ. Sin didn't stop being sin when Christ came, it merely stopped being appropriate for men to take the right of punishment into their hands. Thus the difference between doing away with the law and fulfilling it. Men act unevenly in the application of the law - God doesn't. Thus is fulfilled with evenhanded Justice and tempered mercy that which once was merchandised upon by the priesthood... much like modern fraudulent sects who make merchandise of salvation..