Wow, you are one luck cyclist. Helmets are supposed to break upon impact. The breaking/deforming is a major component of the energy absorption. It is likely that your age contributed to your injury. If you are going to keep riding, I would find a helmet with the most coverage as possible. A skateboard/snowboard helmet may give you better protection, but not a lot more.

I am confused at to why you are on Keppra. It is an anti-seizure medication. Did you exhibit any seizure activity? Some such meds can slow your reactions and reduce you coordination and balance. I would be very careful riding in traffic while on Keppra. Maybe it is just a precaution until you have had longer to heal.

Regarding your mental state, you sound quite alert and functional. There are some online exercises you can do to gauge your functions. Luminosity.com has some free programs as does Positscience.com Google 'brain training' and try some of the exercises. Some are designed to measure driving skills. They can be good indicators of function.

Take care of yourself and go slow. A Second Impact can be deadly.

My best to you.