If significant news is on the table/imminent, is the CEO allowed to continue his 10b5_1 b Share Sale Plan?
Absolutely. That's the whole purpose of these plans - allow for trading at times when insiders possess material inside information. The plan has to be set up at a time when they don't have inside information.
The only gray area is if insiders can cancel the plan if they know good news is coming. That's what the (sleazy) folks at Igen did before Roche bought them.