It looks like Anduin ruined his plans by destroying the bell with the mallet. As for the siege, I believe that is exactly how I believe it will go down. Garrosh will probably recieve massive casualties on his end and the other races will abandon him. Varian and Thrall have been to known to negotiate as well and Thrall will have a portion of his people defect to his side.
if the bell is destroyed by anduin (i really need to do the alliance side of the quest chain and scenario on the ptr)
then either garrosh try to fix the bell or go to plan b and use the sha infused claw and find another way for his troops to control the sha before the sha destroys his troops
or i will be flat out wrong and attrition will just wear both sides down till the final conclusion where garrosh is like adolph hitler in his final days and the siege will really be a full on siege of the city and maybe blizz will give us something spectacular instead of a damp squib which is what im expecting tbh