I'm not great at the techy thing either, but you seem to think outside the box with a lot of your storylines so maybe something bizzare could happen to them. Many of the time travel/alternate reality stories seem to go with transporter incident, alien mind altering substances. Maybe it could be something like they are spending now x-mas together on shoreleave because have no where to go. Perhaps they have a funky drink which causes hallucinations or some bar patron alien over hears there holiday woes and decides to help them feel better by putting them in the alternate reality. Maybe father x-mas visits and decides to grant them their subconsious wish. Aw hell maybe some alien culture wants to test their character so they put them in this situation and want to see what they will do when returned to their own time. How about some aliens are using them as lab rats and pick the past time x-mas based on some other human interactions they may have had in the past.These aliens could want to see how humans have evolved since their last encounter with them back in the 20th century. hence the test to see what they will do once back in their own present x-mas time. just brainstorming sorry for the rambling