That's because mtd4 partition is a shortcut that points to address 0xe40000 from the base flash directly. Reading position 0x0 from mtd4 would be equal to read position 0xe40000 from base flash. So your dump file position 0x0 is in fact a dump from memory position 0xe40000.
You mean on the label or in the tplink's partition? Because after downloading your partition, there is effectively only one MAC present. I think that this base mac address is meant to be derived from for other interfaces, that's why there is only one.
No you don't have to. LEDE doesn't overwrite the specials partitions. So the tplink partition is untouched. But if you mean to find out really how the stock firmware attributes its MAC, that could be interesting. My guess is that it will be A4:2B:B0:EA:11:FA for WAN, then A4:2B:B0:EA:11:FB, then A4:2B:B0:EA:11:FC and so on.
You were so close to find it, tplink + 0x8: 
@MaSt is 27050385 your PIN for WPS by the way ?