Just about everything you ask depends on the quality of the unit.
Yes, most have overload protection.
No, as 12 volts is really as good as it gets. While the -12 and +12 would give 24 volts using the -12 as common your current would be limited to less that 0.5 amp maximum.
Yes, they will drive inductive loads but just like any case when the load is inductive snubbers and filtering should be used to protect the PSU as well as any additional circuits.
You really don't want to run any given rail at maximum. This is especially true of multiple rails and again is a function of the quality of the power supply. This is what is known as cross loading.
Ron