As far as I understood the lightning network is a web of connected nodes that keep balances between them and when Alice wants to send btc to Dave she can update her balance with Bob (-1btc, for example) which will update his balance with Carol (-1btc as well, getting even), who will finally update her balance with Dave. If Dave closed all his channels now he would receive 1 btc more than he would had received before the transactions, while Alice would receive 1 btc less and Bob and Carol would not have their totals changed.
My question is: how do we find this route Alice-Bob-Carol-Dave? When Alice meets Dave they dont know what is the best path between their nodes, or even if there is one.

As far as I understood a LN Channel is open and closed by their participants by broadcasting and confirming transactions on the chain, so they only pay those transactions fees and not all the other they may do while the channel is open. If that is right, the LN would work only for recurrent payments and not occasional transactions, right?

If I go everyday to the same coffee shop it would be worthy to open the channel with them, but not if I'm passing by one and decide to make a one time purchase. It seems that we are putting to much hope on the LN, but it wont solve a lot of problems. Is there anything I'm not getting right?