Seems like you're basically screwed.

Thinking out of the box... Do they need to be 30rd mags? Could you get away with 20 rounders? What about a mag coupler or fast mag to get one of the mags on your rifle and off your person?

You also need to find lighter mag pouches. While TACOs are cool, I don't see a point for them if you don't need to swap mag types. Otherwise a standard AR type mag is awesome.

What about TT fight light or Blue force helium whisper? You have a LOT of heavy nylon.

Get a stiffer, lighter internal belt without heavy hardware on it (like a D-Ring).

Can you find a way to not stack your mags? Having the weight further out from your body will cause excess sagging. Get the Taser on a duty drop and off the MOLLE adapter so you have more MOLLE space available so you won't need to stack.

As for suspenders, what about suspenders under your ACU top (you said MP?) attached to your inner belt, and belt keepers to the outer belt?

He's already said that it's command directed that he keep 3 30rd mags on his belt.

I concur with the suggestions of using lighter-weight pouches, and deleting any double-stack pouches.  Finding room/location seems do-able considering the pic provided.  Again, I concur with the idea of using a lighter, wider inner belt.  The cobra-buckled belt you are using was designed for use (along with the metal loop) for restraint in helos and such.  Unless this part of your mission, some considerable weight-savings, as well as better belt pressure distribution could be obtained by using a more suitable, wider and lighter inner belt, which would allow you to also delete the relatively heavy cobra buckle.  As you probably know, such safety belts are not recommended for rapelling, and for good reason.  Any reduction in the stiffness of the new inner belt can be remedied by using some 1/16" thick HDPE sheeting inserted into the outer belt, and UNDER the inner belt.  Available at Graingers and Amazon.  Measure carefully (templates are great), and cut with aviation snips from Harbor Freight.  Sand ALL the edges, and round off corners, lest the plastic wear holes in the nylon.  That will stiffen the outer belt (and spread the weight) without any significant weight penalty.  The sheeting will also serve to reduce "pressure points" from which you seem to be suffering

The easiest solution would be a better set of suspenders.  I'm a little confused because you mention a "crappy H-harness" in your original post, but later on you mention that adding a pair of low-profile suspenders is not possible.  Perhaps you could expand on that, as well as any uniform or command issues that might be pertinent.

I'm sure that a good solution can be found.  Perhaps it might be as simple as changing where you wear your belt, perhaps moving it up or down a bit.  That said, there is a definite limit as to how loose a heavy belt lacking suspenders/harness can be worn without the danger of it slipping, as I'm sure you know.