When the router's forwarding capacity is exceeded, ingress packets are dropped - the router physically receives the packets but due to lack of buffering they can't be stored.
When the downlink's capacity is exceeded, packets are dropped by the upstream router - your router never sees them.
Both events cause TCP retransmissions. However, TCP's congestion control kicks in and the sender adapts the stream speed that's exceeding link and buffer capacities, so after a short while retransmissions become rare.
When you're downloading from wherever and monitoring the receiver, you won't be able to see retransmissions because the initial transmissions haven't reached you. Only the sender sees retransmissions.
The data link layer has nothing to do with this. The Ethernet IPG (physical layer) is completely unrelated, it's fixed and can't be changed.