In your answer to 3.)b. (above) it seems to me that the total area of the top of the mizbeach that was fit for burning korbonos was not 26 x 26 Amos but rather 24 x 24 Amos because inside the area of the keranos was a 1 amoh walkway for the Kohanim to encircle the mizbeach. This would create a maximum surface area of only 576 square amos rather than 676. And if we would answer that if necessary this could be used b'dieved as a place to burn the korbanos, then perhaps the area of the keranos could also be used b'dieved as a place to burn the korbanos if there was no more space.
Aharon Levine
That is an important point. As I see it, the answer to your question is that in rare situations ashes would be allowed to overflow into the area of "Hiluch Raglei Kohanim," as you suggested. The ashes were allowed to overflow there because they did not impede the movement of the Kohanim, since the Kohanim could still walk in the Amah of the Keranos area, stepping on or over the Keranos to get from one edge of the Mizbe'ach to the next. However, they were not allowed to overflow into the Amah Keranos, since that would impede the Kohanim, preventing them from circling the Mizbe'ach.
Mordecai Kornfeld