1) Should be free! They've not charged for impressions and no one else has either at the events.
2) Some vendors have merchandise to sell and some won't. You can most always purchase merchandise that is on sale with a credit card, or they will direct you to their online shop. The prototypes won't be on sale. Sometimes certain manufacturers will sell a demo if you work it out with them.
3) I would personally go both days, especially if you plan to talk to Head-Fiers (I hope to meet you!) Time flies, in my opinion, and is only good to go one day if you have a plan of only checking out a few booths. With certain shows that are three days (like this upcoming LA Audio Show), I'll be spending two full days there but only an hour or two on Friday to pick up some new gear from manufacturers.
Oh and with regards to just being at CanJam for a few hours, there will more than likely be after-event activities as well. Last year was Go-Karting at an uber-awesome track with hills, and we played table tennis at my current favorite table tennis place in the world: Bounce, which is just a mile or so away from CanJam London!
Hope this helps and looking forward to seeing you there!