Jacob Katzenellenbogen, of Keidaniai - Sources for children

Dear Jeremy, What you are saying is possible. Have you been able to obtain a yDNA test from any of your Shapiro cousins? This could be a crucial piece of evidence to really seal the case.
Cordially, Adam

PS: If funding is an issue, I could help with a portion of the cost.

I can reach out to them. What test is most appropriate?

Leaving here in case useful: I checked in Da'at Kedoshim (Eisenstadt, 1897), and it goes through the Katzenellenbogen lineage down to Yekhezhel. It gives his children's names, but doesn't go further, unfortunately.

The book is digitized at archive.org - https://archive.org/details/daatkedoshim00eise

The most appropriate test is the Y37 test at Family Tree DNA.

I've emailed my cousins. Haven't heard back yet.

This is the type of quest where failure is not an option :-)

I know.

I've emailed my cousins again to bug them.

I've emailed Dr Rosenstein again as well; hopefully he'll have time to answer. Although I really hate bugging him because it's time taken away from his work.

In the meantime, I've downloaded some relevant sforim off of Hebrewbooks.org. Nothing in Markovitz' Rabbis of Keidan. Didn't spot anything in Yesh Manchilim, either. I'm now plowing through Avot Ateret Lebanim, which isn't as accessible as Daat Kedoshim, unfortunately. I wish there were indices!

If you haven't yet checked Utsr HaRabanim, this would be recommended also (has an index): https://www.dropbox.com/s/5r4ijmx6nd4zezu/Friedman_Utsr_HaRabanim.p...

Thank you! I didn't know that one.

I couldn't find anything relevant in Avot Ateret Lebanim.

Alas, nothing in Utzar HaRabanim either.

Looks like no option but to press on with the yDNA angle. You may want to refer to the project on GENI which I sponsor to show that there is substance to the idea: https://www.geni.com/projects/Rabbinical-Lineages-yDNA-and-Haplogro....

Looks like no option but to press on with the yDNA angle. You may want to refer to the project on GENI which I sponsor to show that there is substance to the idea: https://www.geni.com/projects/Rabbinical-Lineages-yDNA-and-Haplogro....

On another related tack: I am working with a co-researcher who is a descendant of the Ba'al Knesset. I compared you to her on FamilyFinder and you have enough matching to so predict with 59% certainty a relationship in one of these categories:
: 6C 6C1R 5C 6C2R 4C1R 5C1R 7C Half 3C2R 4C2R 5C2R 7C1R 3C3R 4C3R 5C3R 8C or more distant
According to a hypothetical tree I built using your tree and hers, you would be 8C2R so that means that the FF result at least supports your working hypothesis.
If you contact me at haplo@xyvy.info I can detail this further.

My cousins still haven't responded to my emails (that's not unusual, they often don't check email regularly). I'll see if I can get in touch by phone next week.

I find that I match quite strongly with any random person with Litvish ancestry (typically at least 60cM), so not sure if your co-researcher's match is indicative or not. I'll email you though.