Beyond usual quantum mechanic description of entanglement, is there any QFT or stringy formalism/explanation of it? Currently entanglement is speculated to be one underlying mechanism of emergent spacetime, but what are its foundations?
 A: It's interesting you are asking this question.  Just few days ago an article was published in the Quanta Magazine discussing precisely this issue.  There it goes beyond what you call “the usual quantum mechanics description of entanglement”.   In the new theory, the quantum entanglement (QE) is applied not just across the space but also across the time.  By doing it and by analyzing its outcomes, scientists are hypothesizing now that the whole space-time might be originated from the QE across time and that the QE is the more fundamental structure of the universe than the space-time.  
This theory corresponds well (not necessarily in QE, but in the interpretation of space-time) with the Transactional Interpretation of Quantum Mechanics as proposed by John G Cramer in 1986 and as expanded by Ruth E Kastner in 2012, in her book:  “The Transactional Interpretation of Quantum Mechanics – The Reality of Possibility”.
The recent article in Quanta Magazine is found here:
https://www.quantamagazine.org/20160119-time-entanglement/
