why did Russia collapse in the first game and hang on forever in the second? What causes Russia to surrender? Was I just unfortunate in the second game?
$0.02 CAD follows.
I suspect at least some of what you're seeing is the depth of the game and that this allows significantly different paths to similar outcomes and vice-versa. There are enough variables so that the game often plays out differently every time.
Imagine that in one case the people rallied around the Tsar whereas in the other, revolution and disorder carried the day. It's for you to decide if the results were in the realm of the possible or not.
Did you adapt a Russia-First strategy both games?
As the German's you have an advantage in that a Grand Offensive need only be started on one front and neither Austria or Turkey are required to conduct one. If the Enver Obstination Event fires you need to conduct two attacks per year with Turkish forces but they need not be planned Grand Offensives. As a bit of history, Enver was a fanatical believer in the power of the offensive and time and again forced his army commanders to make ill-considered attacks that the Ottoman army was poorly equipped to conduct. An Entente player needs to execute two Grand Offensives and for the Eastern Front, these involve capturing the objective plus five-other areas, something that can become virtually impossible using the Russian army later in the war.
Massing German combat power in the East is probably the key to CP victory in WW1G however in real life the Great General Staff never anticipated that the fabric of Russian society was as vulnerable as it turned out to be when they planned their two-front war. That said, Tsarist Russia did survive disaster after disaster and managed to last into the winter of 1917 as an effective ally even with much of Germany's army on the Western Front. In fact both Russia and Austria may prove surprisingly resistance to collapse or succumb to internal collapse fairly early in the game and I would submit that this represents the range of potential outcomes reasonably well.
One of the strengths of WW1G (IMHO) is that after the potential for an easy victory in 1914 has passed, you really have no other war aim than to win at all costs and this reflects the situation that the belligerents themselves faced. Victory conditions in the game are a bit nebulous and sometimes you think that you should have done better than you did but again, look at the actual event and Entente "victory" proved short-lived and hollow with the costs nowhere compensated for by any substantive gain.
Naval historian Richard Hough called the Russo-Japanese War of 1904-05 "... one of the most wretchedly useless wars ever fought."
but I really think that World War One also belongs on that list and that is one of the attractive aspects of WW1G the game.