You don't need to spoof a different MAC or IP to implement your own stack on a raw socket, Python is not too slow to handle negotiating a connection, and your interpretation of how tcp/ip works is flawed. I highly recommend you read a good book about tcp/ip and learn how the kernel works with network applications of different types.
In terms of using Scapy for your packet crafting, here are some guides with examples that may help you work around your issues. (Hint: use the Scapy-defined sending and receiving routines and don't implement your own, or stop using Scapy and implement your own raw packet sockets) http://securitynik.blogspot.com/2014/05/building-your-own-tc... https://github.com/yomimono/client-fuzzball/blob/master/fuzz... https://www.sans.org/reading-room/whitepapers/detection/ip-f... http://www.lopisec.com/2011/12/learning-scapy-syn-stealth-po...