I have less of a problem with the offence than most. I take the view that if you are entitled to claim for an overnight stay, but at your own detriment choose to endure the trip back instead, what's the harm. The employer loses nothing as they believe there should be an entitlement to stop overnight anyway. The employee picks up a small bonus for putting up with it.
That said he's justification regarding staffing seems a little thin: and to clock up £14k seems a lot. At that cost one would expect parliament to provide the accommodation instead. A decent 'expenses' system should be simple enough to provide a cheap but acceptable options and then not need to overly care when it is claimed.
As for gaol, we I think it is another case of public indignation ensuring harsh penalties to those that get caught. Few have much sympathy for the MPs caught fiddling. I'd guess he'd consider a tag is better than staying in gaol for the whole sentence.