As far as I know, MAC addresses are only available when you have Ethernet (802.3) as the data link protocol. Serial connections don't use ethernet. Instead, HDLC or PPP or etc... is used. These don't need mac addresses since they are all point to point.
So simply, there are no MAC addresses for either of the s0/0 interfaces.
Its going to depend on what you mean by Encapsulation.
If you are looking at OSI then each layer encapsulates as it goes down the stack.
At the data link layer, its a point to point connection using MAC addresses. If, at the network layer, if the traffic needs to go to a seperate subnet, at the data link layer a point to point connection is made to the next hop address to then route the traffic. Therefore it goes from Host to Router.
On a serial link, it is using a different Point to Point protocal, if i am not mistaken, the network layer is then encapsulated into the Point to Point Protocol to get sent over the Serial connection, when it is then routed on, it then gets encapsulated using the ethernet datalink encapsulation to get from the router to the destination host.
From Host A to Router A, it would use Ethernet 802.1 and at the DataLink Layer MAC
From Router A to Router B, it would use what ever encapsulation you are using on the Point to Point link (PPP/HDLC/Frame Relay, etc..)
From Router B to Host B, it would use Ethernet again.