I totally agree, everything is far too convenient, but it always will be with stuff like this. If someone you knew turned up as though they had never met you (or at least this incarnation of you) before, your first thought probably wouldn't jump to identical twins.
I suppose the quasi-whimsical happenings in the show are acceptable because they take place in a magical ponyverse, but when it comes to a world deliberately engineered to mimic our own, familiarity makes us notice quirks and abnormalities more easily.
I always wished that sunset shimmer took her henchponies back through the portal and there was a story continuation in that direction but, once again, as is so common in the series, the ending tied up far too quickly.
I think you're right, it was probably quite rushed because of the hasbro execs on the writer's backs, plus I guess it would be difficult for the single writer Meghan McCarthy to get it perfect with deadlines etc.