Ok, first, you need to eliminate either RT301 or RT302 as possible problem sources. RT301 is a 1k Ω variable resistor and RT302 is a 4.7k Ω variable resistor. Both should be adjustable from 0 to the max reading. You might not be able to get max resistance if there is a parallel path, but something is wrong if you can't get as low as zero. If the readings aren't within specs or not what you are expecting, take them out of circuit and check again. No sense proceeding if either of these are failed or out of spec. On post #19, you mentioned something about getting a high resistance reading from RT301. Although that's not the adjustment for pilot signal, the readings that you are sharing does not sound right for a properly working 1k Ω pot.
Then once you've confirmed this, if any pot is/are bad, replace and recheck. If problem still persists, you should consider that it's entirely possible that your MPX decoder chip is bad. Happens a lot.
As for your 19khz adjustment, you should understand that a "true" frequency counter is the best tool. A cheap one, a makeshift one, a computer one, arduino counter, or even a counter on a good DMM won't necessarily pick up the part of signal that is important to you. The reason is that every good dedicated frequency counter will have adjustments not found on cheap ones such as low pass filter, trigger sensitivity, etc. In a perfect world, a frequency from an oscillator would be clean and stable. But in the real world, they often are not. If the signal is noisy, it signal might be unusable for a frequency counter because it is unable to differentiate from the actual part of the signal you want to isolate and capture. You did not set a location in your "profile" so nobody knows where you are located, and I'm not going to waste my time guessing, but since more than 1/2 the countries in the world use 50hz mains power, I'm going to presume that is what your counter is picking up. Of course if you were using a more sophisticated counter, you would be able to use a low-pass filter. I have in the past, found that my fluke digital multimeter using the frequency mode was unable to capture the signal I wanted, but a high quality dedicated one had no problem by tweaking the trigger sensitivity. But is your particular problem because you don't have a good clean signal, signal too weak for the counter to pick up, or is it because your MPX decoder isn't actually outputting a proper signal? I don't know. All I know is that what it's picking up isn't the pilot signal, it's likely 50hz from the mains.
Hopefully, you check the first part regarding ensuring that the pots are actually working properly first, then go from there.