if I try to send "Hi" to device 1 and "Hello" to device 2 and neither one is online, when one of the receiving devices comes online and it tries to send a new packet to it containing "What", it would send "HiHelloWhat".
You're saying that if you submit a packet to one device when it is offline, the shield will subsequently send that packet to a completely different device? That sounds horrendous
I haven't looked at the Ethernet libraries to verify this but it sounds a reasonable explanation.
It appears as though there is a separate buffer for each node being communicated with.