Wonder if we can do something in tandem....and bring in the special maneuvers
Being Bloodied makes them subject to rout by way of intimidation checks (basically the normal rule) == you can also take out enemy leadership which instant bloodies them. This introduces a new maneuver which is very pc like.
You can force enemy movement via either bluff or intimidate and if not threatened perhaps diplomacy or even a heal check to allow allied swarm spending a healing surge (1 per tier) == Things like the Parley trick earlier mentioned might induces opportunity to do so.Though companions already can be affected by pc induced use a healing surge abilities.
Bluff might also have the conferring advantage thing happening at large scale (could these require you leveraging your own swarm to help apply the bluff? and help apply the intimidates? or add modifiers as a aid other on your check so the more swarms you have the better your check)
You might be able to conscript/marshal new troops (heal or regenerate the swarm when your troops encounter an allied town with diplomacy or intimidate if you have already used diplomacy once and variations of these things. This to allow some terrain impacts that are less obvious.
Once you start thinking of really large swarms I find my brain wants broader ways to interact with them and I think having some of them more tightly defined would be useful.
However sometimes you might just need the more definitive super broad impacts like taking out the leadership defined or suggest with additional ones improvised mostly by DM and players and have them achieved flavor wise as individual successes and failures in a skill challenge.
I am not familiar with the arena rules...
Based on what you wrote, I think you and I are on the same page for the most part. Many of the options you brought up that would involve either rallying the troops together or causing enemy troops to flee were things that I was connecting with the morale check. I also like the idea of using your bluff check to cause enemy troops to think you're going to go one direction when you're really going another way. Adds a level of strategy to the combat field and since for each combat is tactical to begin with, the most you'd have to do is just make sure that the units have the specific combat actions you're looking for the same way a particular monster would. You might even actually be able to take a solo monster and just reskin it as a unit.
One thing that I've really had to keep in mind when I'm coming up with my home brewed stuff is thinking about it from a story perspective before a tactical perspective. What sort of stuff on the players going to run into that's going to help drive the story and the combat forward. Watching movies like Braveheart have been useful for this for me. At one moment, your people are standing ready as a swarm of monsters are coming at you and next thing you know you're completely surrounded by chaos. You might be fighting for 3 rounds straight and then get a lull which will allow you to reassess to see where things are at and then you might fight for another four rounds before some champion of the enemy units sees you and decides to call you out and mark you in single combat all the chaos of battle continues around you. This will be the start of the thing where a Combat Challenge (a collection of smaller combat interactions that drive towards a specific goal or objective) would come into play. There's different strategies that the allies will use and there's different actions that the enemy units will use in relation. Coming up with an outline scenario to help the DM keep track of it all while the players are engaging on their side, at least for me, helps keep it straight.
As for the units, I found so far, that using the monster rolls in the monster manual has been useful in regards to their function in combat. Particularly the skirmisher and the solder. Brutes and controllers I find usually end up being independent agents on the field or in smaller clusters so having a swarm version of them doesn't make much sense. One of the movement actions that I had from my soldier units was Reformation where they would take their move action and restructure how they are formed. Skirmishers on the other hand, are kind of nebulous by nature so having a specific action to reform them didn't seem to make sense. Also, I had both my soldier units and skirmishing units have artillery as a secondary role that they could take up if they wanted. It depended on whether I wanted to be frontline fighters or if I wanted to be in the behind the front lines shooting volleys of arrows or other things at the enemy.
As for the arena combat rules, they were introduced in an article called "Fight!" Written by Robert Schwalb (I love all his stuff) in one of the Dragon Magazine articles. The article was also republished and Dragon Magazine Annual on page 75 if you have access to that. I believe the article also talks about pit fighting and one-on-one combat when you're surrounded by NPCs and other monsters watching you go at it with your enemy. In the area of combat, most of those folks would be distracted by people attacking them so if anything, if you hit the edge of the arena there is a possibility that you might be attacked by another monster.
So that's the stuff that I've been playing with regarding Mass combat. Like I said, I haven't had a chance to really have my party deal with a large army confrontation, but I was planning on having more of that come up once they got towards level 20 and above. Right now, they're dealing witha bunch of Hill and Earth Giants in the Revenge of the Giants Adventure trying to keep them from releasing a primordial before redirecting them back into the Tomb of Horrors. We will see how things go.