I am pulling out a 3way switch and replacing it with a smart switch. The second 3way switch will be dead. Since the load is in the second box, I am using one of the travelers to connect the switch to the load. I have run into a few odd things.

1)A box has 5 black wires nutted together. Two of them go to the same switch. Is there any conceivable reason for doing that?
2) That switch also has 2 other black attached to it. One goes to a second switch next to the first switch, I have no idea where the last wire goes. Essentially they are using the switch as a junction. I am changing that, but was it code compliant? (I vaguely recall that either a switch or an outlet can't do that, but don't really remember)
3) One light is set up so it uses a hot from one box and a neutral from another. They are on the same circuit. I vaguely recall that you can't do that with metal boxes because they set up an induction loop which can get hot and start a fire. There is no metal except at the panel, and since they are the same circuit that should be fine. So, is that setup okay with plastic boxes?

Thanks.