Possible, but will require additional logics.
As indeed does option 1, it's just that the logic is called an "Arduino"
I'm inclined to say use two Arduinos, the comms between them is simple (depending on the speed required) where as muxing the SD and Ethernet will I think be harder.
So one Arduino does nothing but sniff the SPI and forward the data to the other.
The other handles that as well as the sensors and ethernet.
As long as the average data rate of the SPI data is < the rate the 2nd Arduino can offload the data to the ethernet that will work.
So I think it boils down to the average
data rate on the SPI. We've determined that the bit rate is 4MHz, how long are the bursts and how frequent?