There is no definite solution to your problem - you need a 4.1 SHSH and you don't have one - it's now too late to get one.
The only idea that I have (as - yet untested) -
Use this technique Mostly iPhone hacking: Data recovery: not just for iBoot-pwned devices to make a 4.2.1 IPSW that upgrades instead of restore.
Then restore to that. The check that is causing you problems is ONLY in the restore ramdisk.
Then you can use tiny umbrella to exit recovery mode.