It has been almost 10 years since I got rid of my last W126 but I do remember the model quite well given how long ago it was
If you have rebuilt the monovalve and checked the rest of the system, it is almost certainly the climate control unit. This assumes your engine warms up properly (which means the thermostat is opening). If you aren't getting heat with a warmed up engine, it's most certainly the CCU.
Some things to check first.. is the blower motor working fine? If you turn it to cooler air and put the blower on high, do you get a rush of cool air? It could be that you have heat but since the blower is dead none of it blows into the cabin. First make sure the blower is working fine on high.
If that's not the problem, considering that winter is coming I'd look into getting your CCU rebuilt by a reputable company. I think most can do it in a week and ship back to you.
There are also CCU units on eBay for $50. You might want to buy one (or two!) and hope that one works perfectly. You can always keep the other as a spare, send it to be rebuilt, or just re-sell on eBay if you don't need it.
Check with the rebuilders if they have one on the shelf ready to go. They may sell you one with a core charge, then you send yours to them so they can rebuild it. These panels are pretty common so most rebuilders should have them.
That way you only do one swap.. you get a rebuilt one shipped to you, install it, then pop yours back in the same box and mail it back. Easy to do.
I'm sure there is a way to jump the potentiometer/rheostat ("temperature wheel") so it sends the full hot signal. But you'd have to look at the wiring diagram to figure it out. Might be more trouble/risk than it's worth. Even then, your unit could still receive the full hot signal from the temperature wheel, yet it's broken enough that you still won't get heat.
If the blower and thermostat are good (engine heating up properly) then I think this is the last part that you could realistically consider to be the problem.