Woah! Time for some clarification! Mongols were trained in use of composite bow and riding from a VERY young age! You are right that cavalry is at a disadvantage due to forests in East Europe, but The Mongol invasion of East Europe went flwalessly despite that. They routed Hungarians, Poles, Germans, and Lithuanians while only controlling a very small army and caused havoc in the countries. Many think that it was just a raid because their army was so small, but many in Hungary found themselves living in forests because their country was defenseless after Bela's loss to the Mongols. Mongol scouts also reached the walls of Vienna. Oh that brings me to soemthing else. The Mongols were ALWAYS outnumbered! That garbage you hear about them riding in 200,000 strong is due to inaccurate reports from the enemy. They would trick their enemy by putting dummies on spare mounts and they would also light extra torches at night to decieve the enemy. You are right that they were all warriors, but there were not that many of them. Most armies they brought to battle were around 50k at most and the ones brought to Europe and Russia were half that. There loss at Ain Jalut occured because they were outnumbered about 1:5 or 1:6 and caught off guard. The Mameluks brought their entire army expecting to meet the main Mongol army, but instead found a very small contingent of troops, so it was their main army numbering hundreds of thousands vs. like 20 thousand. That thing you said about them not using formations well is also incorrect. They were the most organized force in history up until Napoleonic times. They organized troops based on groups of 10k, 1k, 100, and 10 with 10 being the most elite troops. This mosr advanced than anything Europeans would do for a while. Their system of officers also resembles modern times. As for you saying that in CK we will face Mongols who are not strong, that is incorrect to. Mongol troops would remain an effective fighting force until the mid 1600s. Any Mongol you come across in CK will be a great warrior. Their were ill disciplined ones, but that was mostly Chagtaids and it was due to disorder in their society. When Tamerlane ruled over them they were probably one of the strongest armies in the world.
Snall, to answer your question technically Mongols should always triumph, but you guys are correct they they could not expand into Eastern or Central Europe permanately because not many horses could be supported. The Russians should always be subdued by Mongols, but I think the developers might come up with alternatives. Maybe The Mongols would be forced to turn back even earlier?