RE: Penalty for not using car seat?
Your local police precinct office or state troopers office would be able to tell you with a quick call.
The penalties vary but can include fines and mandatory safety seat classes/workshop. My brother was ticketed in Florida for have a SnugRide forward facing (what can I say, my brother is not the sharpest tack on the bulliten board) with a toddler riding in it and the penalty was a fine and a class that he had to pay for.
Still, you really needn't worry about justifying it to them. My parents and grandmother are very much a good part of my life and while they may not understand that something is needed, they respect that I get to decide for my son. Perhaps the problem would be solved by having a spare seat to keep at their house so you can still do things spur of the moment?
Katie, mama to a pair of boys.