Was hoping Rob would jump in to answer your question, Nick.
I can't answer it all as I'm not enough of a sparky. But I do know a DPDT stands for "double pole, double throw". It is a type of switch, one can get them at Radio Shack and the like, various styles. Essentially, if I have it correct and if I don't I'm sure someone will correct me, these things basically switch two leads.
Most light switches, for instance, are single pole, making or breaking the hot wire. A double pole on a 110 circuit would do both the white and black wire.
As for the relay, when 110 power is supplied to the coil, the contacts connect both 220 pairs. So really doesn't matter which side is line and which is load as when the coil is energized, connection is made straight through for both of the 220 leads. Trust you've viewed the photo Rob imbedded a few posts below.
Sorry I can't help you much on the bin full sensor. From what may be faulty memory, essentially what Rob was doing was installing a lamp and a photocell, so when the bin is full, the photocell isn't being energized by the lamp. The photocell Rob specified was one that "turned off" when light was broken, essentially the opposite of your standard photocell night light.
In other posts, Rob noted he is, or at least was, quite active in another woodworking forum, and if you can't raise him here, might be worth a try for you to attempt contact there. Or, and maybe easier, this forum allows one to email another member, without divulging that member's email. The original senders, as I recall, is transmitted to the recipient so that person can answer you. His screen name is McRabbet.
Regards,
DWD
Regards,
DWD