To answer one of your questions: any Google employee who has administrative access to the hosts (file servers) on which the drive data is stored will have read-access to your data: this includes the operations engineers, service engineers, system administrators: these type of employees are typically the ones with that type of access.
The answer to your other questions depends on what methods Google uses to remove distributed data across multiple hosts, and whether they are using any backup methods to store copies of the data. Nobody other than Google technicians will know the answer to this question, and of those probably only the engineers who maintain the filers and the data distribution structure.
To give you an idea of how truly "privy" that information is: I used to work at Yahoo as a Unix sysadmin, but I didn't work with the filer infrastructure and so even I don't know what the answer to that question would be for Yahoo data, although I spent a year working in the operations group for two of the departments there.. Only the engineers directly responsible for that infrastructure are going to know if the data is truly "deleted" or not (and to what level of deletion, if that makes sense to you).
most likely, the file system pointers to the data are deleted within one or two days (depending on how long it takes to propagate through Google's infrastructure), while the actual data itself continues to reside on hard disk until it is overwritten: meaning the data (or parts of it) could theoretically (and fairly easily if someone made the effort) be recovered many months later.
Regarding other agencies or companies with access to this data? Nah .... the only way the NSA or somebody could access it is if they have employees working at Google. Unlikely. And even if they do, do they have root access on the servers in question? Not likely. As someone else mentioned, theoretically the government can get access through a subpoena process. But from a practical perspective do these people have willy-nilly access to John Anybody's data? Nah.
The only people who theoretically intercept your data when you upload it are the system administrators and operations engineers who work at the companies that provide the network backbone for the internet: such as UUNet. I know for a fact these people for many years have scanned internet traffic for illegal content, because years ago when I lived in Virginia I interviewed with UUNet for a job which involved this work. They offered the job to me, but I didn't take it because I accepted a job elsewhere at the time.