SAM still works to fix push notifications. It just doesnt work to actually unlock the phone anymore. Doing that will fix your battery drain.

If you can get 5.12 baseband back on there you can make a custom 4.2.1 with pwnage tool or snowbreeze and restore to that to preserve your baseband.

4.1 did not come with 5.13 baseband. 4.0/4.0.1/4.0.2 did.

Alright, downloading iOS 4.0 firmware, hopefully the phone will preserve the 05.13.04 firmware on restore.
Can I later update it to iOS 4.2 preserving the 05.13.04 firmware?

Yes you can. You may have issues restoring to 4.0 though without SHSH. Thats why I suggested 3.1.3.

I tried SAM before. Even if it does fix push notifications, it still has to deactivate the phone. And since I can't activate it officially with SAM, I have to hacktivate it again so it's unlocked, and the phone will then again keep looking for push certificates.
Alright, I'll see if I can restore it to 4.0, if not, what's the worst case scenario?
I have 3.1.3 laying around already.
I recall having huge issues installing custom 4.2.1 after 3.1.3 to preserve the 06.15.00 baseband. iTunes gave the 1600 error multiple times, and even with iREB I could not get to white screen. However, somehow I did it eventually after entering recovery mode in iTunes and going in to DFU on iREB.

You arent using SAM right then. You dont need a sim card after deactivating it. If you know what carrier your phone is locked to choose that carrier under country and carrier, then pick the SIM ID that shows up there. Then plug into itunes and itunes should activate it. You will know it has worked when under SAM it says wildcard activated under activation state. Obviously you will need to revert lockdown to stock and deactivate first. But trust me, SAM will work to fix push notifications. I do it all the time.

When restoring to a custom firmware you have to enter pwned DFU first. iReb can do this but I prefer to use redsnow to do it, more reliable. Redsnow 0.9.6rc19 should be able to do it for you.

I was using everything right, even without the SIM card.
The guy I got the iPhone from said that it was originally from Ireland on the O2 carrier, and I could see that it's region was Ireland when checking it on some program (can't remember which anymore). So I manually set the country to Ireland and carrier to O2. It didn't work. iTunes was thinking for quite a while, and then said the same thing, that I have no SIM inserted. I also tried all of the rest Ireland carriers, none worked.

And these are the effects of this Why is iPhone Unlock with SAM not working?

Were you picking the sim id when choosing O2 Ireland? Sometimes choosing by bundle instead of country and carrier works better.

I might have not taken the sim card out yet when I tried to activate it by bundle. Might try it later on.
Alright, iTunes gave me 3194 error when trying to restore to 4.0, however, I tried to dump SHSH with iFaith and it dropped ''UNSUPPORTED DEVICE Aborting...'' error.
Must be because of the corrupt baseband. I'll install 06.15.00 back and drop SHSH then.

3194 is a no SHSH error. With iPhone 3G you can get around that with pwned DFU. You dont need a baseband to dump SHSH. Either use pwned DFU or restore to 3.1.3 like I suggested.

I go into pwned DFU every time I restore an FW through iTunes. It still gives me the 3194 error.
But good, I'll restore to 3.1.3 this time.