BTW, how will the costs for opening a payment channel be determined? Who decides on those fees?

One has to get a bitcoin transaction mined to be able to open a channel.

The cost for a bitcoin transaction depends on how much people are willing to pay for it.

There is currently a fee market with a fixed supply (1 MB block chain space every ~10 minutes) and variable demand - an often-used analogy is a bus with a fixed number of seats and people bidding for getting a place on the bus.