In response to your two questions:
(a) How could only certain parts of the site be lost?
This is highly possible if the backup they restored for you is way way back the current state when the servers go down. Its highly possiible that these are new pages you uploaded recently and if their backups are say one month ago, it would be lost upon restoration.
(b) Is it reasonable for me to expect a hosting provider to restore a website to a working condition following the loss of one of their servers?
If they say in their terms of service (TOS) that they are responsible for restoring your website upon server crashes, then you can expect that. But looking at most TOS of hosting companies nowadays will tell you that you, as webmaster is responsible for your own hosting backup. If thats the case, the host can get back to you and say you shoulld have kept your own.
These are lessons to learn in being a webmaster. Always remember to backup your websites like you are brushing your teeth. Its one webmaster job you cannot simply miss amidst adversities with your webhost.
Hope this helps. Cheers!
