why is double-spend not an issue for payment channels? Micropayment Channels (MPC) cannot be used currently because of transaction malleability, I hear. 

AFAIK, people saying that have in mind that the refund &amp; payment tx, which have as input the deposit tx (Dtx), could be invalid if the deposit's hash was changed. I m fine with this. 

What I don't understand: if the attacker manages to modify Dtx' hash before it made it to the blockchain, I believe (s)he can equally manage to double spend the btcs supposed to go to Dtx. Thus, double spend seems as much a threat as malleability. Yet no one discusses it, so I suppose something is wrong with this reasoning. What? 

Is the following correct:

whenever malleability attacks are possible, then so is double-spend attack. 
?

Thanks
Great question. In order to understand how it works, you have to understand that opening a micro-payment channel (MPC) requires a commitment before you get started. That is, both parties have to show proof that they can't double-spend these transactions before they get started.

How is this possible? SCRIPT, of course. A transaction (2-of-2 output) that's dependent on other transactions is signed first, which opens up the MPC, and then, and only then, do both parties release the transactions that would make this other transaction valid. At that point, the funds are committed to the channel and cannot be double-spent since it's locked in a 2-of-2 output, which must be signed by both parties, hence eliminating the possibility of a double-spend.

A more thorough explanation can be found here.