Switching back and forth from 12v to 48v
You could just use the 48V as the only power source and then PWM it down, using a FET, to ~12v (25% duty cycle) and then increase the PWM when you need more power.  But beware, as  Tony Stewart Sunnyskyguy points out, that you could overheat the motor.  Time spent at the higher power should be carefully controlled.

To address your numbered items:


The dead time between relays could cause the motor to lose torqe and the claw would release.
As you mention, it only goes up to 46V. I'm not familiar with motor drivers, there could be one out there with a higher rating.
Do not use a linear regulator. It's just going to cause you heart ache with power dissipation and possibly other failures. There is too much power going on.