I really like it a LOT!!!

I have these 2 comments:

You state:
Our cats usually ignore the spare room for a couple of days, hoping that the new cat will leave !!!

I actually find that the resident cats are drawn to the door like magnets to metal!!

my point is that there are many ways the cats could react.

and
leave the door ajar and let him start to explore the rest of the house and allow him to encounter other resident cats.

Good step except usually prior to allowing the new cat out I invite the resident cats in, one at a time, for introductions so the new cat is not taken by total surprise when he exits.