MCL injuries are the more common injury (as are med. meniscus...lat. meniscus is more rare), and their mechanism of injury is quite different. MCLs can happen just by catching an edge, by losing an edge, by changing direction quickly, etc. It doesn't even require a fall. Same with the med. men. injury.
MCLs and med. mens. are independent of each other, though you sometimes see both in the same injury, though not commonly in my experience. You'll often get someone with an acute MCL, and MRI reveals an old med. men. that they didn't really know they had ("Oh, that's what that pain has been for the past 6 months"), and most just assume they are from the same injury.
ACLs lead to instability in the knee, as do MCLs. However, MCLs have a higher propensity for healing themselves, and very rarely need surgery. ACL injuries are probably more common than reported, but a mild, low grade sprain to the ACL will also heal itself. The complete tears are what get all the attention, and surgical repair is most often required, especially in the athlete. The rehab protocol is also much more involved for the ACL injury.
It's kind of like auto accidents vs. plane crashes. A lot more people die each year in auto accidents, but when a plane crash happens, we all hear about it.
Lots of different studies regarding arthritis, and they all seem to be contradicting each other, so who knows. It does seem that genetics plays a highly important role in arthritis formation, even acute stress induced OA.
Seems like the other info presented so far is pretty accurate.