Mark 16 is going to be largely a repeat of all the questions from Matthew 28 (please check that one out as it explains a lot), but I know all of the resurrection accounts have their own quirks that need to be dealt with, so let's see what Mark brings up as new.
"Were the men or angels sitting or standing?" Yeah, this is a contradiction, but a pretty minor point. If I had to give my opinion, I'd go by the consensus of Mark and John's Gospels I guess. I was going to say because they were eye witnesses, but they weren't eye witnesses to this event, were they?
As the SAB points out, Mark's Gospel is largely believed by scholars to have ended at verse 8, making verses 9-20 a later edited better ending. This is probably true, as I understand some very good scholarship points to this being the case. The SAB goes on to address the last verses, however, so I do as well. And anyway, it reads like an add-on as it doesn't seem to stick with the more or less chronological story so far, but adds some notes about the day as a whole.
"Did the eleven disciples believe the two men?" I think this is a manufactured contradiction, as the verse in Luke doesn't say the two men were believed (and I do believe that this passage is talking about the two men from Luke, which I'm sure we'll hear more about when I cover that Gospel.) So they said Jesus had appeared to "Simon", but are we talking about Simon Peter, the apostle, or some other Simon? (You may have noticed that "Simon" "John" and "Mary" are very popular names in the N.T.)
There are a lot of other questions that I've answered in various places towards the end of this chapter, but the last one I see here being new is "When did Jesus ascend into heaven?" I suppose this may be a contradiction, but there are a few things to be said about the whole thing. As I already said, the last few verses of Mark aren't about establishing any kind of chronology of events, so they can be dismissed for this question. With that out of the way, only the passage in Luke 24 remains to be addressed, as the other verses, although listed separately, really do agree. (I mean, forty days is many days, isn't it?) So what is there to be said about the Luke passage? I would say this: There's no reason why Jesus couldn't have ascended into Heaven multiple times, after all, he came back down from Heaven to witness to a number of people long after Pentecost, most notably the Apostle Paul.