I'm pretty sure when Peter broke his concentration while casting the spell that Strange accidentally said "Save Martha". That's guaranteed to **** up any universe.Yeah, how though?
The spell does or doesn't work, Parker needs some better examples and Strange takes him on a journey of guidance to meet other Spideys. The sling ring can take you to other verses, but how you grab a bunch of villains from their place in time and space makes me wonder. It is Marvel, so it will probably be some dumb justification.