I'm doing a TL about the Khazars as well, and I can help if you want (lots of research done on them).
Were you talking to me or the OP?
Let's go back to the more famous later khanates-
What if one of the Mongol royal family converted to Judaism? There was Sartak who was Nestorian after all, and also Faaelin's Prince of Peace had Orda Khan becoming a Christian. If Mongol high generals and direct descendants of Genghis could be Christians, why not Jews?
Already been don - see David bar Elias's "Jewish China."
The effect is going to be even bigger the Magyars emigrated together with the Bulgars, so we will likely not see a Bulgar Kingdoms in Thrace.
So some effects:
No Magyars, the Hungarians plain (the Hungarian enclave in Transylvania) is colonised by Bavarians, they were only thinly populated (200000) by Moravians (Slovaks) and Avars before the Magyarian invasion, the Magyars doesn't serve as outher enemy for East Frankia, so the we could end up with the tribal duchies, becoming independent states, with the whole Frankish Empire (the German tribal duchies, the Kingdom of Lombardy/Italy, and a disunited France) loosely united under the HRE.
No Bulgars, a stronger position for the Romans in Thrace, we could see a succesfull Hellenisation of the south Danubian Balkan. So a stronger East Roman Empire. Together with the fact that a large part of the Turk becomes Jews, we could also see them keep Anatolian.
Lesser contact between Rus and Constatinoble, maybe the Russian doesn't go Othodocs but becames Catholics instead or the different Rus nation become different religion, with the North going Catholic, while the south becomes either Othodocs, Jewish or Muslim.
I see several problems with this:
1. Etelkoz was the area in Ukraine and Moldova which the Magyars migrated to in the 9th century, around 200 years
after the Bulgarians migrated to Thrace. So the Magyars staying in Etelkoz will not stop the Bulgarians from migrating 200 years before the POD. However, I would agree with you
if the POD involved the Magyars not migrating at all.
2. I agree with your point about Bavarian colonization. The Germans were already in Pannonia (the Balaton Principality was under their suzerainty IIRC), and without the Magyars settling I think they would have continued to migrate there.
I'll have to disagree about the decentralization point. At least with regards to Italy, I think without the Magyars then Italy would have become a very powerful state. During the reign of Berengar I of Italy, the Magyars invaded and devastated a large part of Italy. To defend his kingdom, Berengar was forced to grant huge powers to the feudal lords which made them independent in all but name. Before this Italy had been relatively centralized.
I'm not as sure about Germany, but IMO it was pretty decentralized in this period anyway (Susano or any other Germans, want to help me on this?). The Magyars only served to decentralize further, so without their devastating raids Europe as a whole will be a lot more centralized.
As an aside, I'm not sure were people got the idea that the Magyars or Vikings caused Europe to centralize. In the 9th and 10th centuries, the greatest force causing the spread of feudalism was the raids of Magyars, Arabs, and Vikings causing the central rulers to delegate powers to local nobles in order to defend the entire realm.
And about the religion: for my timeline idea, I had the western Rus' eventually becoming Christian, while a second Rus' state along the Volga (centered at Rostofa / Rostov) becomes Jewish due to extensive contact and trade with the Khazars along their main trade route.