If GC expires before N-400 processed, will holder be illegal?

[Boysnberries]

If my dh's GC expires before he finishes the N-400 process will he be considered illegal? We tried to file 6 months in advance as recommended, but the fees were holding us back (dh has lost his job 3 times in 3.5 years); we considered filing a fee waiver, but decided it would go against us, and ended up having the money gifted to us. His GC will expire 4 months from the time of applying for citizenship.

Where does he actually become a citizen for purposes of legally working; is it after a positive decision has been made or at the oath?

Thanks in advance for your help!

 

[sanjoseaug20]

GC expiry and PR status expiry are two different things. GC has an expiry date, PR status does not.
He becomes citizen on completing oath.
The biggest trouble if your GC expires before oath is that you can not travel outside the country. If that's not an issue and the gap between GC expiry and oath is not so long, don't worry about it.