Older engines had a smaller bore in the carb, and would draw fuel fine without the extra pressure from the muffler. The addition of muffler pressure then enabled a larger bore in the carb to be used and thereby gave a higher power at full throttle.

I have never tried to measure the pressure from the muffler at idle, but it is there (even if it is small) and it will help the weak fuel draw from the carb alone at low rpms.

Reading through your posts again, I think perhaps you simply have the throttle set too low. Most .15 engines will not idle reliably at 2500rpm, as you have indeed also found out. I only ever had one .15 engine that would do that and that is the Norvel .15 which has a very restrictive stock carb.

On a .15 engine you'll be fine with anything below 5000rpm, with around 3500rpm at best I would say. First tune the high end needle at full throttle, then tune the airbleed screw for a nice throttle respons. Note that this is richer than what will sound ideal at a stationary low throttle setting, but it a safer tuning that will ensure that the engines doesn't hesitate upon acceleration. After that the idle is what it is, don't change the airbleed setting, just use the radio to set a reliable low rpm. That is from where you can advance the throttle fairly quickly without any hesitation from the engine. Don't try to use a too low throttle setting, just use what is reliable for the engine, i.e. it shouldn't die if the plane is tilted a little (±20° roughly) and it should respond well to the throttle. I don't have any of the .15LA myself, but I would guess that the reliable idle is in the range 4000-5000rpm, and anything lower is just an added bonus (with 2500rpm being unreliable in practice).

Here is an example of a carb tuned for best in-flight performance, it is an OS .10FP (with the same muffler as your's is) I was really surprised at how low and reliable the throttle is, but it is far from 2500rpm: