Q1. The time taken for the first bit to reach first hop: 1000/2*10^8s. Time taken for transmitting 10000 bits : 10000/1M Secs
So, by the end of `10000/1M + 1000/2*10^8 sec`. the whole packet will reach the router.
As you said in your question - assuming there is no processing delay in the router, at the end of the before said time, the router starts transmitting the packet. And from the router to destination takes the same amount of time as source to router. So at the end of 2* before said time, destination gets the whole packet.
Q2. The only difference between first scenario and second scenario is based on the store and forward logic you mentioned in the question. That means, the router is able to transmit only after the whole packet is received. so it has to wait the previously calculated amount of time to start transmitting. But in the second scenario since the packet size if 1/4 of original, it can start transmit the first pkt at the end of 1/4 of original time. But in both the scenarios the time at which last bit is received on the destination will be the same.
Does it answer your question?