I disagree on the order, and you left out a very important aspect; Hardening the home.
My version of priority:
-Early warning system: Either electronic or 4-legged (preferably both)
-Hardened exterior: Doors and windows that cannot be easily defeated in mere seconds.
-A plan: Whether it is egress, retreat to a safe room or standing your ground, it needs to be decided ahead of time.
-Weapon: This is an absolute necessity when the first three measures have failed
-Cell phone: Land lines are easily cut. 911 is unlikely to prevent an attacker from getting to you, but you will almost certainly need to make that call at some point during or after the invasion, especially if you or a family member are injured
-Training: this is last not because it isn't important, but because it is the least likely to be done and pretty much useless without the others. Being some kind of martial arts master or having all manner of other HtH training won't help one bit if the intruder is armed with a ranged weapon and has distance on his side.
All that said, many an untrained person with weak barriers, no alarm and no training has effectively repelled or ended an attack with a firearm. While using a gun most effectively requires good training and practice, simply using one is a monkey task. As well, most invaders are not Navy SEALS or otherwise highly trained operatives. The visions of SWAT-like teams of criminals sweeping your homes with tactical equipment seems a common one here, but it is extrememly unrealsitic; I don't know if we can even count the number of times an intruder has been shot by an untrained armed homeowner as he carelessly came through a door or rounded a corner.
