An LN Hub can place a time lock on any Segwit Infected Blockchain coins.
Even those which which are not part of the hub's channels?
LN Notes are like Bank Notes, completely worthless unless someone is willing to trade you something for it.
There are no LN notes. LN will be transacting BTC.
LN transfers happen Offchain , so their is no verification by the actual blockchain, that what you are trading for even exists, until you cash out.
Verification is very easy for channel participants. It's as easy as checking an address in a blockexplorer today.
Say Bob and Carol have a channel with current funds distribution Bob - 3BTC, Carol - 2BTC. Let's represent this as
Bob-3:2-Carol
Alice wants to pay Carol 1 BTC, but opens a channel with Bob (he plans to build a hub, charges no fee for now), she loads 1 BTC to the channel. So now our small piece of LN looks like:
Alice-1:0-Bob-3:2-Carol
There are 2 channels: Alice-Bob with 1 BTC in it and Bob-Carol with 5 BTC. Bob owns 3 BTC: 0 in his channel with Alice and 3 in his channel with Carol. Alice owns 1 BTC, Carol - 2 BTC.
Now Alice pays 1 BTC to Carol via Bob:
Alice-0:1-Bob-2:3-Carol
As expected, Bob still owns 3 BTC: 1 in his channel with Alice and 2 in his channel with Carol. Alice couldn't pay more than 1 BTC. But now she can use her channel with Bob for receiving up to 1 BTC. For the whole lifetime of a channel, this channel's BTC never leave it.
If now, due to a 51% attack, Alice-Bob channel is no more, this directly affects only Bob, he looses 1 BTC, Alice gains it and can spend again, effectively performing a doublespend, just like with good old onchain transactions. Carol isn't affected, she has all info about her channel with Bob. Bob's problems aren't her problems.