you do need to lock your bitcoins, that way LN would trust you to then do offchain tx's.
That locking would be done using CLTV not using the mempool (so the locked funds *are* in the blockchain as a tx output).
Using the mempool would be rather silly as that isn't in any way guaranteed.
so if the locked tx IS in the blockchain.. then those 5 customers, 1 starbucks =6 locked tx's in the blockchain (500byte each=3,00bytes)..(ignoring employee as he just a recipient)
then when starbucks finally closes the channel.. another tx appears in the blockchain 30 days later (1,250byte as the example on the right of my diagram).
so now 5 purchases of coffee are not 1,500bytes(old way left diagram) to pay everyone in order and give change.
but now 4,500bytes to cover the 5 locked tx's and the 1 settlement tx to pay 7people (5customers change, starbucks, employee).
i think LN has its uses to finally create more secure means to handle bitstamp day trade or a poker room 'pot'. but im still trying to analyze the benefit for the one off retail market.