The funds you deposit at the beginning can be used to pay multiple seeders. You have one "ledger channel", funded by that on-chain deposit, which is being used to open a "virtual channel" for each seeder you connect to.
Not directly. State channels are similar to lightning, but there's a key difference:
* in lightning each payment is routed through the network independently, requiring participation from the intermediary nodes
* in a state channel the intermediary nodes are used to establish a channel and then the payments are direct
The upshot is that in state channels, once the connection is established, the payments are truly peer-to-peer, and don't need any interaction with a third party.
We use this property in Web3Torrent, as the payments are sent directly peer-to-peer on top of the webtorrent messaging layer. This wouldn't be possible with lightning, though you could probably find another way to accomplish something similar.
There's a bit more about it in this post: https://blog.statechannels.org/channels-funding-channels/.