Multiple "to be"

I've noticed what appears to be a merging of two verbs "to be" in Indo-Iranian languages. On the one hand, there is the Sanskrit root "as-" ("ah-" in Avestan and Old Persian due to RUKI), which is an existential verb "to be." In addition, in Sanskrit, there is the root "bhū-" which may mean "to be" existentially but also in the sense of "to come into existence."

I've noticed some similarities between the Indic and Iranian sides of the tree here. The conjugations of "bhū-" ("bhūvaiti," inter alia) seem to correspond to the New Persian literary/poetic conjugations of the corresponding verb "to be", "budan": "bovad," etc. These conjugations are used fairly infrequently, again, mainly in poetry.

Likewise, the conjugations of "as/ah" seem to correspond to the New Persian *short* copula of "budan": -am, -i, -ast, -im, -id, -and. These are used in regular speech.


Both the long copulas ("hastam, hasti, hast," etc.) and the "regular form" ("mi basham, mi bashi," etc.) seem to be "bureaucratic" back-constructions (per Thackston)

A few questions arise[*] from this:

- Why did the verbs appear to merge into one?
- Does the {as-/ah-}/bhū- merge have any relation to English irregularity of "am/is" and "be"? "am/is" seems to be related to conjugations of "as/ah-" and "be" corresponds to "bhū-".
- Is there any relation between this bifurcation of verbs "to be" with the spanish differentiation of "ser"/"estar"?
- If so, it appears to be a relation of *meaning* as conjugations of "ser" roughly match those of "as/ah" (cf. Latin "sunt" with Avestan "hƏnti", factoring RUKI), but not *morphology.* In such a case, is "estar" a "back-construction" similar to the New Persian long copulas?


[*] which means I'd be using bhū- ;-D