Let's back up a tiny bit.
Unlike something like ProTools, or Cubase or Sonar, MyDMX doesn't really recognize or acknowledge the MIDI interface. While something as simple, cheap and durable as a MIDISport 2X2(bought in 2002, been with me through Win98, SE, ME, 2K and XP, sorry, no Vista!!), MyDMX isn't that complicated. It merely listens to whatever MIDI information is running through the system bus, regardless of where it is coming in from.
Basically, if the OS is aware that there is an MIDI interface connected to it, MyDMX can look at it. You don't have to tell MyDMX anything. MyDMX is aware that "there is possibility for MIDI" and that's good enough. Since MyDMX isn't active in engaging the MIDI interface, it behaves differently.
So, if you want to go with a MOTU unit, you should be able to use it just as readily as something much simpler and less expensive. I've never been a fan of MOTU hardware, but that's a big throwback legacy issue to when MOTU was just a pain in the butt and went out of their way to be difficult and I've never forgiven them. Still, let your application requirements be your guide. Remember, you're going to need at least TWO MIDI interfaces: One on the machine running MyDMX and another on the sequencing computer.
Now here is something you brought up about tempo sync. MyDMX does NOT respond to MIDI clock or MIDI Time Code(with or without SMTPE). This may become an issue if you're going scene-based triggering in MyDMX. Hear me out. When you program a scene in MyDMX, you always have the choice of using or NOT using a fade to transition between scenes. This can make switching look relatively cool an "seamless", but this may inhibit what you're trying to do. of course, you don't have to use a fade. But, fade is in time, not in time code. Faster sequences will NOT make it do the fade faster.
Another option might be to manually transition via CC information the various DMX channels using your master scheme of direct control in the sequencer. While complex, this solution does give you total control. As in, if you slow down or speed up, the CC data output by the sequencer will speed up/down as you adjust your sequence's tempo, hence properly time adjusting the CC data. This can guarantee perfect sync. Granted, it is more work and YES, it means potential bottlenecks and slowdowns on that MIDI cable, but that is assuming you're loading a ton of stuff on there. Lots of CC data will quickly load up the wire.
Universe: in DMX, this is the assembly of all your lighting and their associated channels, into a single cable's worth of instructions, not to exceed 512 unique DMX channels. Some higher end consoles support multiple universes, as does even the Elation software product, Compu Live. MyDMX, in order to be cost-effective, only supports one universe at a time. This is obvious when you look at the MyDMX dongle and note that there is just the single DMX output.
In DMX, there are no rules prohibiting the duplication of channels. So, I could take my 8 64 LED Pros and just address them to the same channels and they'd all act exactly like each other. Since these are 7-channel units, I have the optin of using up to 56 channels(addressing them differently) or just 7 total channels by addressing them the same. This is a good way to "cheat" without necessarily givng up anything.
MyDMX is relatively simple in regards to MIDI. It's not picky and SHOULD work with just about anything. My MIDISport 2X2 is USB, the MIDI on my M-Audio Firewire 410 is the firewire bus. If I can finally get that used Digi 002 surface with PT LE 7.4.2, I'd also have MIDI on the firewire port once again(with cool motorized faders).
Going back to using a lighting controller, such as a DMX operator:
Even with sequencing to get your "tempo sync", like with MyDMX, there is no implementation of clocking. If you have the fader for fade set too high or too low, it can alter your fade times. While the computer may say "OK, go here NOW" and the scene fade fader is at 2 seconds, well, the controller will get the message, but take a full 2 seconds to fully transition from one scene to the next. You also have to give things up like moving scenes(can't be done) because there are no steps. The fader setting may not be perfect for all songs and is not accurate enough for reliability in such an environment.
Neither solution is perfect. Some have more strengths than others. I think myDMX might be the better directions.
Now, using the free download on the ADJ site, you can start playing with MyDMX. The only thing you give up is being able to control lighting since you won't have the dongle/DMX interface box. Even so, you can fully play with MIDI triggering and even the 3D Visualizer and start designing your show. Should you wish to buy the dongle, then you only need to plug it in and launch DMX and you're up and running. Demo mode basically means "no interface". The software will not time out or expire. This is especially important for REAL light designers(like, not me!) because they can design their shows from the bus, truck, airplane or where-ever, then when they get to the gig, plug in, and rock and roll.
My suggestion for using 1 scene with many many steps for a song does have its drawbacks. You're going to have to adjust your timings of steps to hit your spots. This COULD be time consuming. Any mistakes and adjustments will impact things further down the line. If you just take it "one at a time" though, you should be able to pull it through. The key is a stopwatch with a split function so you can measure approximate time between steps. This just means work. Sounds like you're not opposed to work.