Safe parameters for external nerve stimulation using electrical impulses
Nice question(s). I will go through them in sequence.


Safe current limits. With regard to electrical stimuli it is current density that determines safety limits. Therefore, in medical devices, it is not a matter of what is a safe maximum current level, but what is the maximum safe current level given the surface of the electrode? And in terms of skin stimulation, it is not the safety limit that is the bottle neck, but the maximum tolerable current density, which is around 0.5 mA/cm2 (Prausnitz, 1996). Higher stimuli result in pain sensations. This is one of the reasons why the BrainPort device is used on the tongue; the moist surface reduces impedance and hence the currrent needed to elicit a tactile response.  
Permanent damage. As said, the tolerable sensation limit is more of an issue (a must-read in this regard is Kaczmareck et al., 1991). 
Acuity. The best-possible tactile "resolution" is generally expressed in terms of tactile acuity. Tactile acuity varies across body sites, and unfortunately it is about the worst on the thigh (45 mm) and the back (40 mm). A much cited picture on tactile acuities is the following from Weinstein (1968):




These acuities represent the distance in which 2 simultaneously presented, static pressure points need to be spaced to be just noticeable (the so called two-point discrimination threshold). Although your proposed electric stimuli are different from pressure stimuli, these reported values in the figure (40 - 45 mm) give an impression on the inter-electrode distance needed. From my own work I can confirm the ~40 mm using vibratory stimuli. The unfavorable acuity of the back and external bodily skin in general was also one of the rasons to put the electrode grid of the BrainPort on the tongue, which has excellent tactile acuity.   

References
 - Kaczmareck et al. IEEE Trans Biomed Eng 1991;38:1-16
 - Prausnitz, Adv Drug Delivery Rev 1996;18:395-425
 - Weinstein S. 1968. In: Kenshalo. The Skin Senses:195-222. Springfield, IL: Thomas