persistent air in hydronic heat pipes

For reasons I will spare you the details of, I have to figure this issue out myself after paying a lot of money for 4 plumbers to come and provide no advancement. If you are reading this in RI, please contact me for this job! I have no experience with these things, but have been trying to educate myself over the last week, so hopefully my descriptions here will be intelligible.

Gas fed boiler installed about 2.5 yrs. ago. I noticed issues with hot water this summer and circulator pump was replaced (with a different brand/model), not by original boiler installer because he was not returning calls. Everything was fine for a few months, until the first heat zone call, when I heard a loud whooshing noise (like a burst of compressed air being released) which gave one sudden pulsing bang to the pipes and the sound of gurgling water through the pipes after. That is what typically happens now when a zone call opens a zone valve. This never happened before the installation of that pump. Two people have successfully purged it (the original installer during install and someone shortly after who worked on some rotten pipe sections that were found) and so it seems possible to do so, but I was not around at the time they did it and so I don't know how they succeeded.

(no cap on boiler auto air vent b/c one of the plumbers verified that it was loose then dropped it into the boiler, so need to buy new one)

I don't think it is a leak because system when shut off will hold pressure steadily for the 10 hours that I tested it, and I have checked for water anywhere that might appear, although there are areas behind drywall in the finished basement that I could not see.

I learned how and attempted to purge it myself a few days ago. The plumbers here were not letting each zone run very long and were opening up multiple zones at once, so I wondered if doing it more methodically and longer would help. I did the following:

-turned off system and let cool for 1 hour

-shut off the valve at the purge point (yellow handles in pics below) to block flow to the return of the boiler; paid attention to the direction of the T on the bolt on the side of it so that it was open from above (as opposed to open from below if the handle is turned the other way, if I understand this correctly)

-manually opened zone valve, then opened purge spout with hose attached (and run to drain at washing machine)

The fill water comes in, but with the purge spout completely open, there is only 4 psi registering on the meter. All of the videos I've seen show more pressure during the purge and the ability to use the manual override (or fast fill) on the water feed PRV to push it to 20/25, being sure to stay below 30. I can only build up to about 20 psi by partially opening the purge spout and then using the fast fill to get more pressure. The plumbers before opened the purge spout completely and did not build up pressure and I never heard air come out of the system. When I did it this way, I did hear air come out and let it run for about 10 minutes per zone, well past the time of not hearing air come out. I did zones 3 (indirect water heater), 2 (finished basement), 1 (main floor) individually and paid attention to isolate each side of the split loop on zone 1.

However, still having the same issue. Here are a few of my HYPOTHESES:

1) Maybe it is necessary, after purging the zones, to then move the cutoff valve at the purge point (in pics above) to the opposite direction, aligning the T on the nut to allow flow from below and to then open the purge point valve and/or the drain valve at the bottom of the boiler. Maybe air was left in there. After I purged and then turned on the system, I heard air in the circulator pump as soon as it turned on after the first zone call. If I understand correctly, where the feed is coming in would form the open loop for purging starting there and ending at the hose, leaving the pipe below it unpurged. Am I right about this?

2) The feed PRV is maybe bad? However, when filling the system again, it does its job of stopping it at 15 psi. The overide lever does work when the system is closed and will increase pressure in the system, so it at least works for that. But could a PRV malfunction cause not enough feed water to enter the system during purging? I've checked the water pressure at a couple of spouts and am getting 60psi there and not much difference between hot and cold water flow at faucets upstairs (noting this because the boiler feed also feeds the potable water in the indirect water heater).

3) Something to do with the expansion tank? I checked the pressure there with system pressure at 0 and the tank was at the pre-charge of 12psi. Feed PRV valve is set at 15. Should they be equal? Would this contribute to at least the pressure of the air slamming through the system? When filling the system cold, the PRV stops it at 15 psi. Then, over a minute or so, it creeps up to 17. With the system running hot, it gets up to 20. Leaving the system not running overnight, it holds at18. When a zone valve opens and the rush of air is heard, the needle on the gauge jumps quickly to 22/23, goes down to about 18, then settles at 20 (this all happening within about 1 second).

4) Perhaps system is sucking in air due to it being piped so that the circulator is pumping toward, rather than away from, the expansion tank, causing air to get sucked into the air separator on the top of the expansion tank (or perhaps into the vent at the boiler) due to a negative pressure. This seems unlikely to me because the system worked fine until the replacement of the circulator pump. Maybe this circulator pump (Grundfos UPS15-58FRC) is stronger (although it is at the low setting now) than the previously used Taco007e, which causes this? Below are the charts for them (Grundfos first, then Taco007e). I honestly don't know enough to be able to tell if there is a big difference between the force caused by one or the other.

This also doesn't seem likely, though, because no air issues during the first few months after the circulator replacement when just zone 3 (hot water tank) was working. It all started when zone 1 (1st floor heat) opened for the first time in October. It seems that this is the least likely explanation for what is happening. Related to this--could air be getting sucked into the circulator pump if it was installed incorrectly? However, for the same reason as above, it seems this would have caused issues right away rather than it only starting when zone 1 came on for the first time.

5) Could the air separator have gone bad? It seems it should be removing this air if it is working correctly based on the distributor's demo videos I have seen of the unit on youtube. Or, located where it is in relation to the circulator pump, maybe it has never done anything but didn't need to because the system was purged properly. I checked and the port where air should come out has the screw turned loose so it should be open.

6) Another idea is that maybe the installer and other plumber after who did purge it successfully soon after it was installed were using some method to increase the water pressure through the system (like a pump of some sort) that I didn't see because I wasn't around. Certainly these recent plumbers have not been using any means of building the pressure in there up to 20. Any advice on doing something along these lines would be helpful. Maybe when I am purging doing what I described above, it is better to get the pressure from more water running through rather than building it up by decreasing the size of its exit at the purge spout.

If you've made it this far, thank you immensely in advance for any advice. I'm concerned these surges are putting pressure on piping joints, the sound is shocking and annoying, and air in the system is bad regarding corrosion and efficiency for sure.