Well, my current setup, while not perfect, allows me to deal with quite a lot of what is thrown my way.

I do still use IWT swarming and sort of just learned to estimate when I can't spam it for the toons to not run through the target. It works well enough for a lot of situation and for trash it's just too much of a hassle to be positioning them again without it, after every miniscule mob dies. For situations where that does not work reliably, I use a Mapped Key Step State Action to "turn off" my IWT spamming and just use a manual press for more control. Similarly I can toggle off auto-assist in my rotations, so I can send my DPS toons to another target while my Tank stays on another. This has proven fairly effective on Serpentrix, so I kill the other spawning head fairly quickly when the boss submerges and tank stays on the boss to be able to interrupt.

As far as the healer, I've not found much use for having it utilize IWT movement, so my IWT key is only ever being sent to "other" toons in my Melee-DPS and Tank action target groups. The only time the IWT would come in handy is the push back from Dresaron in DHT, but I simply swap over to my healer on that to keep it moving forward during that. I usually just drop it off somewhere safe-ish by breaking follow and then only move it when needed.

For side stepping mechanics I've also elected to use an action target group based solution. WASD+Shift/Alt/Ctrl all correspond to moving different groups (everyone together, ranged+healer, melee toons) so I can move them without affecting the other toons.

I'd love to hear what others are using. Perhaps I can pick up something to add, or tweak. :P