I am afraid the answer to all of those questions is NOPE. ISIS did grow and will continue to grow as long as there is foreign military existence on Middle-east soil. It is extremely easy to recruit people to do anything while their national/religious pride is being pissed on by their historical enemy.
IS grew out of the vacuum of a civil war in Syria and is predicated on creating a Caliphate. They cannot physically grow if they are not able to gain ground. Their recruitment message is likely to be significantly weakened if they lose territory and battles. Unlike AQ or even the Taliban of recent years they are not able to avoid pitched battles, they can't melt into the background - they would have to stand and fight for their territory - thus they can be defeated.
They can definitely hide in the population. Rakka still have 200,000 people living there. I have no idea which part of that population are hardcore isis member : 40-50k maximum maybe. Assuming you could take Rakka, you can't just execute or deport every one with a weird accent. Mosul is even bigger.
How many ground troop would it take to occupy all these cities ? This would be a repeat of the second Irak war, or Afghanistan, with the endless terrorist attacks, IEDs, etc.
Even if you have contributions from Kurds, Irak regular army, or Hezbollah, they would not be welcome as savior in Sunni's territory.
Like in most conflict, the military option can be used to create conditions for a political resolution, but you have to provide some sort of long term plan for Sunni population : you can't just tell them they will go back under Assad rule they despise in the first place. Currently, I don't believe there is a clear political solution. In this condition, the military option is doomed to fail.
My earlier post posited not caring, let the civil war continue, hand it over to the "nicest" Sunni group we can live with, give it to Assad - none of those outcomes involve a terror exporting aspirant state controlling territory. If we temporarily control a swathe of territory it puts in a position to make a bargain with someone.
I'm not saying it is good, but is it better than a pretty shitty situation. Do the other groups regularly crucify civilians and hurl homosexuals off buildings?
If we acknowledge we can't impose a new government can we create a new normal whereby we smack groups which get out of line but otherwise stay out of the way. It's actually fairly similar to the old Roman system on the outskirts, and the same approach the British initially adopted in the early days of Empire. It's what the Americans sort of did in Afghanistan before the invasion of Iraq effectively dragged them into a civil war.
