Problems
a) Unlike Egypt, that had an unified pantheon, the iranian world was NOT Zoroastrian during the Seleucids. Mithra, for example, had to be accepted in the Zoroastrian pantheon merely because of his popularity during the Persian/Parthian era. No way could have the Seleucids create a sincretic religion using greek and iranian ideas. Of course, they used Cybele, which was a local syrian godess, but it was pretty impossible to take Mithra and Ahura Mazda and accomodate them. The Romans managed to approach these religions with success (just think of the popularity of the Isis/Mithra cults) because they simply had no pantheon: just a bunch of gods that walked among other older gods.
b) The Seleucids would never leave Syria for Babylonia. The Mediteranean Sea was the richest area of classical antiquity. For geographic reasons, it was pretty impossible to maintain Mesopotamia without the control of Persia, so when Parthia managed to take Iran in the 160/150s, it was end of the road for the Seleucids...
c) Your best bet for a Seleucid survival (even if I don't believe in the possibility without a Carthagian victory in the first or second punic war) would have been for Seleucos I not to be murdered in 281, while he was preparing to occupy Macedonia and Greece. Let him live until 275 and when Parthia will come a-knocking, Syria would still retain Macedonia, Greece, Asia Minor and Syria, a good base for any empire of that age.