The Signature dolls usually are connected to the Core assortment number. That assortment number currently belongs to Monster Exchange, but has never been connected to Art Class. Once they started using the movies to introduce characters (in their signature looks), they started theme-ing the core assortments (Picture Day / New Scaremester / Monster Exchange) - so those would be what I consider new “signature” looks. 

No facts, but she seems to be a late add to the assortment (think Rollermaze Clawdeen and Skull Shores Draculaura). With the playset being dropped, apparently, there likely was some sort of shuffle. I would assume it’s just a matter of time.

She was late to the assortment, however the only stores I heard of carrying the 2nd wave within the US was WalMart - but not long enough to receive shipments with Robecca. She only made it out Internationally, it seems.

The only pattern is that there are 2 seasons in the toy world. Spring, and Fall. Also typically, at least with Monster High - Spring items start showing up in Winter (and hit full distribution by Spring), and Fall items start showing up in Summer (and hit full distribution by Fall).

The store exclusives tend to show up at stores a couple months after the mainline releases, though that's not always the case. Fall often has more exclusives - because it's expected that all of those items will be on shelves for the holiday shopping season.

Boo York, Freak du Chic, and all the recent survey leaks are part of Fall 2015. Meaning you likely won't be seeing any of them till around May.

November/May tend to be sweet spots for the first releases to show up each season, but because toys don't have hard release dates (most of the time), it could go either way.