5 Mbps bandwidth indicates that the downstream speed is 5 Mbps. The upstream speed is unlimited. Downstream bandwidth is the amount of data outbound from the ECS instance; upstream bandwidth is the amount of data inbound to the ECS instance.
See ECS Price Overview.
No. By default, ECS instances in different regions cannot communicate on the intranet.
The inbound traffic is generated by ARP broadcast packets. Each ECS instance is assigned in a large network segment. Therefore, after an ARP broadcast packet is sent to request an IP address within the network segment, your new ECS instance receives the request packet, which causes the inbound traffic. But your ECS instance does not reply with an ARP packet if its IP address is not requested.