I'd be a little surprised if anyone here has an answer to your DALT shelf life question. Not a lot of DALT users here. I'd suggest that you might get a quicker answer by sending a private message to dlnmerced, who is a member here but hasn't posted much lately. If you click on the envelope icon, and start typing dln in the "To" box, I think it will autofill with the rest.
Alternatively, the Facebook group where DALT was most strongly popularized will probably have a fast and reliable answer for you. (It's a closed group, so if you're not a member you'll have to ask and then wait to be accepted. I think that's a fairly quick process.) The group is called "Cluster headaches (trigeminal autonomic cephalagia)"
Hopefully I'm wrong and someone here can answer you.