A bit late to this party, but in re: the granddaughter/math debate. Is the assumption this movie takes place in 2019/2020 sound? Maybe it's in "the near future", which gives more flexibility to the lineage. The remark about there having been no ghost sightings in 30 years loosely references GB2, but it doesn't have to.
That being said: movie looks fun, although it might be more appropriately titled "Stranger Things: The Motion Picture".
Yeah, I can put it on my shelf next to ‘T.J Hooker: The Voyage Home’.