Ok, in case anyone wants to know whether Allegiant does, in fact, cancel subsequent legs after no-showing on the outbound, the answer is no. They said they don't care if the flights are flown in sequence and don't cancel return legs.
With one data point here, what they say is true. I spoke to my sister-in-law today as she was boarding her return flight (the one way she had to buy separately). She said she did OLCI from home the day before the 'outbound', which was technically the return portion as she did not fly the ticketed outbound flight. No problem. Showed up at the airport, checked a bag, boarded and flew and no one said a thing. She actually asked an agent about it today as she was boarding her flight home and this agent also said they don't cancel returns when the outbounds are missed. Go figure!
So it all ended well, other than being out the cost of the original outbound and the other changes she had to make. And she has sworn off Allegiant for life now.