I'm not sure where to start, but I'll throw out some things.

Some systems have a 5V DC supply. Some have a 12V DC supply. Trying to use different voltage systems together is an open-ended research project that's best done if someone is paying you a salary, not by the hour. It's best to use parts that uses the same voltage as your HDMI stuff's IR output. I know Xantech uses 12 volts and I've heard that others use other voltages but Xantech handles everything I need.

The output of an IR sensor (the IR buss) provides enough current to light up several LEDs, so a resistor is put in series with the buss to limit current to each LED. This controls the LED's light output, too. Those adjustments you see on some IR blocks are like volume controls on the LEDs, except they are rheostats instead of potentiometers. I'm leaving it at that for now: they adjust the light level.

So the difference between an IR emitter signal and an IR buss is that current is limited in the IR emitter signal.

You can use diodes (google "diode OR" and be sure to look at the images) to mix together IR emitter signals. You can do the same with IR buss signals. What won't be effective is trying to mix IR emitter and IR buss signals.

Digest and report back on what you've been able to apply to your circumstance. It's late and I might be reporting or I might be rambling....