The "second coming" of Yeshu.
Is this a possibility? Yes, I think it is. Is it "necessary" to Christianity? no, I don't think it is necessary to the religion/belief system of Christianity. Yeshu gave us commands indicative of how we are to approach the New Covenant, we are saved through Eshu, already. Will the second coming be perceived as the 'first' Jewish Messiah? This is a possibility. I don't think it likely that this would be something to remain a matter of confusion, however, that is not in line with the Scriptural indication, imo.
Israel as the new Kingship
I assume this is likely, however, I'm not sure it is necessary.
Is Judaic belief 'sinful'?
No, I don't believe Judaic belief is sinful. It merely is a following of the First Covenant. That being said, is it in 'opposition' to the New Covenant, no, I don't believe so. The New Covenant is adhered to the old, the following of the new Covenant is in essence following the Old Covenant as well.
Is the Jewish Messiah at odds with Yeshu
No, I don't believe so. The term "Messiah" is viewed in slightly different manner by different groups, in essence the beginning of the New Covenant was enacted and set regardless of a Second Coming, or, A Jewish Messiah.