When attracted by the first bomb, the presents go out of the window and up to the breach in a curved path. Magnetism works in a direct straight line. The presents would not be able to get out of the homes.
Barry say there are thousands of people at the tree lighting ceremony but, there are clearly only a hundred or so, nowhere near "thousands."
There's no explanation for how Patty got her hands on a thigh-restraint canon to use on The Flash. It would be something Cisco would build, but if not aware of it as something so specifically designed to anchor The Flash, he would have "vibed" her intent and stopped. However Cisco developed the B.O.O.T. to help aid the anti meta-human task force and as of about episode 2 of this season flash of two worlds. Patty was a if not the only member of the anti meta-human task force so this is how she would've gotten it.