Slow cooker lid gasket smell
Well first off, most slow cookers don't have a gasket.  So its not a problem for them.  Rubber can often times take on strong smell.
I would consider buying one without a gasket - I don't think its a majorly useful feature personally.  Or, consider buying the more versatile enameled Dutch oven (there are some very affordable, excellent ones) .
Yes, most don't have a rubber gasket.  And it would solve that problem.
Yes overcooking will lead to bad/different smells, even in good food otherwise.  It will often lead to bad tastes and textures as well.