Worth pointing out you don't need to open a new direct payment channel. If there's enough critical mass in the network and as long as both parties have pre-existing channels (other retailers, friends whatever), then a route between can be found.
Yes of course. That's one very important point that often gets overlooked when arguing about LN's viability.
single once off transactions, if you wanted to. The network will be less congested, because most micro payments will be done
off-chain. This will make single once off transactions much cheaper. {Can someone verify if I understand this correctly?} IF
single on-chain tx's is not possible, then I am with you on that.
Single on-chain transaction will still be possible, just as before. It's not a change of the Bitcoin protocol, just an addition on top using existing Bitcoin scripting capabilities
While transaction throughput will be heavily increased with LN's increasing adoption, single transactions will likely still stay relatively expensive, at least long term in a scenario where a huge part of the global population transacts money using Bitcoin. Short to mid-term I could very well imagine single on-chain transactions becoming cheaper again, just not as cheap as LN transactions, naturally.
As mentioned above, yes.