I used to love this game, but the enemies swing through each other to hit me. I feel like it didn't do that at first, but no matter...
There are a few ways they can swing through each other (and these are ways you could swing through your allies as well). The first is the overhead slash, if the character is just a tiny bit further behind the friendly in front, his overhead slash will go through his pal, and so can yours. The second way is simply people slashing through the corpses of the recently slain. Timing and observing the person behind the one you're attacking to make sure you're not going to be hit after you kill the guy in front of you and his corpse turns incorporeal is vital to mods like Nord Invasion where going up against numerically AI by often by your lonesome is central.
It makes melee combat against a group maddening. 3+ people can be swinging at the same time, with different attack types, so blocking is literally impossible. And if I swing, one of them probably blocks me!
That's just the game is designed to work mainly. Going up against 3 dudes all at once is a very hard thing to do when they're equal in skill to you, and even more so when they're just better in terns of equipment and stats. The character the player controls is mostly the same as the the ones he's fighting against, so you're all on more or less equal footing than other games like Dynasty Warriors and makes ganging up on powerful people immensely effective.
All I could say to combat these situations is to combat these situations and make sure it doesn't happen in the first place. Maneuvering you opponents into a rough line will help immensely even if two of the at the front can slash at you. Keeping an eye for your surroundings is also vital to make sure you're not ganged up on from random sides.
As for blocking their attacks, it is possible, you just need a shield. The left sided slash is one of the fastest and most easily connectable attacks, so they're generally useful in these situation as well. Also keep in mind that side slashes don't travel through living enemies, sometimes you don't have to worry about an opponent for a little while when he lines up a bad slashing in a scrum like that.
Failing that, any tips for Vanilla? I'm going with a strength build this time since party size seems to be based mostly on renown instead of Leadership. I haven't played in a while, but it seemed like the other lords always had much larger parties than I even at level 15-20.
Party size is also dependent on the title you hold, with being a lord, marshal, and king boosting your maximum party size. Leadership is more important for the reduction of troop costs more than anything, charisma is not a stat that has many other advantages other than the two skills it feeds. As for strength, it generally in itself doesn't pay that much past 12 or 15 strength. Agility and intelligence will pay far more handsomely latter as well as being generally quite good for the agility stat. Being able to backpedal faster than your enemies can run forward is an immensely powerful advantage if you're into hand to hand fighting.
I vaguely remember being in full high-quality plate with a massive mace, and wrecking face through all the blocks (particularly in sieges). Maybe I just need a good block-smashing weapon to deal with hordes...
Your best bet with dealing with hordes is to out maneuver them, either on foot or on horse, preferably on horse since you can't predict whether the horse is mounted or not. Crush through is generally unreliable, relegated to the overhead swing animation, as well as only being available on the slowest shortest weapons. It's great for troops that will die anyway as it assures that they will get kills before they die, but it's generally not very useful for someone intending to live until the end of battle.
Horse combat has always seemed time-consuming and finicky. Looking directly at the enemy seems to make me swing way low. I'd rather snipe with a bow, then engage the rest with a mace.
It takes time to get used to, but it pays a lot of dividends once you reach proficiency especially when you master poking with a lance. It however takes good ol' practice and is not something that can be covered up with stats. Generally try to take note about just what it is you're doing wrong specifically and try to shoot in the other direction. If you're over-slashing a lot, try under-slashing instead of fine tuning from an end that isn't improving.
For me I don't look at the foot troops I'm slashing at directly, and that might be an issue for you. You also don't need to look at the troops you're fighting on foot either if you want your slashing to connect faster! Slashing on horse back depends a lot more on your kinetic sense of where your weapon is rather than on your visual sense.
Other than that, some people tell me that they learn a lot from seeing people who do know how to do it fight. It might help to look at youtube videos of someone who knows how to fight on horseback well in the game and demonstrate it. Likewise, in PvE mods like Nord Invasion, and Full Invasion there are masters of bot slaying that play on those servers that you can learn a lot from.