Looks like horse nettle, Solanum carolinense. IME, comfrey is not easy to start from seeds--seems to prefer propagating vegetatively. That might be why you ended up with weeds growing where you planted them.
Over here, all the comfrey I've come across is a sterile cultivar, 'Bocking 14'. Apparently fertile comfrey can easily seed its way out of control. Maybe someone in your area could send you some Bocking 14 roots? BTW, very small root pieces will grow, so I avoid planting it where I'm ever likely to dig. If it's not disturbed, the only spreading it does is a gradual increase in the clump's girth.
The Russian Comfrey(bocking 14) I'm growing does have the tiniest spines that will stick in your fingers if you're not careful. They are so small I often times don't even realize I've been 'stuck' until later in the day when something rubs across where the spine is stuck in my skin.