Stellar is also a worthy contender although it wasn't designed with the purpose of payments, but decentralised currency/crypto exchanges.
The Lightning Network daemon codebase on Github is a beautiful well commented piece of Golang code though.
Not really. DAG/lattice based cryptocurrencies offer different security guarantees compared to blockchain based ones.
I'm not sure what domains see this problem crop up but I've seen it in quant finance strategy execution. Say you have 1k strats that you want to run in parallel but risk needs to bound the bank's per-equity positions globally. When the strats work on different subsets, DAG approaches aren't a problem, but when most of the strats trade APPL/IBM your once very wide (parallel execution) graph narrows significantly (becomes more sequential) as the strats need to check with risk w.r.t. AAPL/IBM sequentially.
NB: for this reason, causal approaches are pretty rare to see today (though it depends on the domain as high latency contexts don't really care).
Nano is also interesting, but limited. In terms of moving value around, I think Stellar is more compelling.