Instead of putting urbanite and rocks in the cob, put containers of water. The temps should be low enough that you could even use plastic. Water can hold much more heat than rock and would conduct well encased in cob.
Air doesn't compress like water expands, it's much harder to compress air, the expansion of the water won't fill the air void, it will just displace it causing a fracture. You might be able to put frozen water bottles into a cob structure while building but as the cob dries and the water melts the cob would have a tendency to settle in the voids left as the bottle contracts.
That being said I think it would be effective in a number of conditions, If your water never freezes and never boils you'll reduce extreme expansion and contraction issues. Keep the lid up and do leave a 1/2"air gap but more to mitigate the cob pushing in on the bottle and pushing some air out of a less than sealed bottle. It would only happen at the beginning as the cob will have a tendency to contract to a degree. Water bottles will expand to roughly twice their size before they burst so that's not an issue.
If the bench is only receiving radiant heat due to solar aspect (in an enclosed area) it might not matter at all, the mass would prevent freezing and without heat impact it should last for the life of the plastic, I would use soda bottles over water bottles for strength.
Back to the questions:
-Would a straw blanket around a portion of the bottle near the top help to allow expansion in extreme cases.
-Why not add an anti-freeze agent to reduce the possibility of expansion; automotive, salt or something similar
-Wouldn't heat melting the bottle be more of an issue over time than the effects of freezing. A 0.5 % failure of the bottle per burn could cause it to fail in 1-3 years depending on the climate. You could keep the bottles further away from the burn chamber so there is less direct heat and they only receive radiant heat (I like this concept).
Definitely an interesting thought experiment which really needs sample data before use. You would have to be careful to protect the bottle from damage from any rocks or urbanite in the mix.