K, I'm fine with everything until #4, and after that.
There are, at most, probably [and I'm being generous here] 15 portals available in 25 man. So, at best, half your group can enter the portals. That means the other half dies.
The buff from the portals lasts about 35 seconds-ish. So, even with a reasonable respawn timer of the boss of 15 seconds, you still have half your raid dead. The first set of portals, though, doesn't pop up for at least another 15 seconds, so your stacks are thoroughly gone before you can use them.
My question is, have you actually -achieved- this far-fetched idea? If not, lol, try it.

If you do get it to work, rest assured you'll get kudos, but having experienced the fight personally in both 10 and 25, this plan fails.