1) 1.2.1 does say that "These steels may be welded to any of the steels listed in the latest D1.1" I agree it's pretty vague but the fact remains.
2) I dont know what the significance of the 1/2" material is, but it shows up in all of the qualification tests where sheet steel is being welded to supporting structural steel. I asked a SCWI the same question when preparing some WPS's for fillet welds and he "suggested" using a range of 1/8" - 3/4". I know this does not answer your question, but it's all I have.
First of all, why are you using a E6022 rod? I assume you are using the SMAW weld process. In D1.1 you should be using a E7015, 16, 18, or 28 for ASTM A 572 grade 50 mat. Look at table 3.1 in the D1.1-2002 code book, it gives you all the filler metal combinations for prequalified base metals. Yes it would then also qualify that filler material for group I and II, I hope this helps you.
This was a WPS that was submitted to me for review. I was told that the PQR could be done on a Group II steel since the electrode had to match the lowest tensile strength of the two steels being joined together. In this case being the decking. I rasied the flag about table 3.1 and after talking with our SCWI he said this would be ok.
Part 2 of your question, It qualifies you to the material tested and thicker, you might have to do a seperate PQR for the .125" material. Now, if you would have done your PQR to sheet to sheet, you would be qualified to both sheet to sheet, and sheet to supporting structural member.
Where does D1.3 state this? I havent been able to find it.