Some inspectors walk the roof, and some don't. Typically your inspector has you sign some paperwork that spells out what he does, and what he doesn't do.
Your inspector said that the shingles were "newer than the home"... what does that mean? Are they 5 years old... or 25 years old? Perhaps the shingles were fine, but the roof was rotting beneath some newer shingles.
Personally, for an inspector who doesn't walk the roof, I would hope that he would get inside the attic. I understand that access was tight... but an effort should have been made to do one or the other.
Liable...? I doubt it... I'm sure his paperwork, which you signed, seriously limits any liability to the fee that you paid. Good luck.