Aimee, probably the only ones who will make any money on this deal is the lawyers for you, the inspector, the roofer, your insurance company, and the former owner. Each of which could possibly be assessed a small percentage of fault. However the problem was not there prior to the storm so it is doubtful you would get much fault out of the inspector. Remember you knew you had an old and sagging roof before you bought it and the "let the buyer beware" principle applies. For example, you can't buy a 1966 ford pickup and then expect to recover cause the seats or suspension sags a little since it was new.
The causation of your problem is going to be the storm damage for which you need to contact your insurance company and file a claim. "But for" the storm, you wouldn't have much damage. A lawsuit would also be looking to see what you did to mitigate any damages such as clearing snow off the roof, etc. so before it is all over you will get caught up in some of the blame. Good luck with the insurance company. If they think the inspector or others is at fault, they will go after them.
Also, an interesting post would be one from your mother on how often, how much, and for what did they pay shingle claims on. I think it would be an expose. I keep hearing stories placing blame on the contractor for not installing them properly, or paying only for the shingles based upon the small percentage of remaining guaranteed life (without paying for labor), blaming it on storm damage which is an insurance company problem, etc. That story would be enlightening and may we will fall over after reading it.