As I can see on the steps above, there are few things I want to point out first.
1. Ireb basically a tool to help you to put your phone in Pwnage DFU. You can do that without Ireb if the ultimate goal is to downgrade the 3gs to 4.1.
2. When you have a baseband of 05.16 on your phone and you downgrade it to 4.1, you will get a 1015 error, the phone will go to recovery mode. On your step above, you mention to ignore it and then turn off the phone. It is impossible to turn off the phone when it is in recovery mode. So, please clarify.
3. Again, the second time to restore to 4.1, you did not mention the error at all and how you get it out of recovery mode.
Anyway, I will buy a 3GS and test this method as soon as I get a hold of 1 and report back.