Holley Street Avenger 570 Idling Problems

Opening the secondaries can be helpful on radical cams, but in your case I think they just went to the jar of likely answers. It got worse when you put it in gear because the engine was making even less power - way too lean.

As far as getting full vac on your ported source, Holley tech was logically guessing. The port is located just above the throttle blade's idle position. At idle the nearly closed throttle creates the pressure drop which is the manifold vacuum. As the throttle opens past the port, the port becomes exposed to the manifold vacuum.

At this point, my suggestion is start over because too many things have been changed.
Always good idea to checking the fuel pressure and (if the avenger has sight hole) correct fuel level in bowls. (Higher fuel level will make slightly richer).

Adjust the throttle stop position (aka speed adjustment) so the transition slot looks square below the throttle. If you don't want to take the carb off to look, then back the screw off until it doesn't touch, then turn in whatever number of turns the instruction sheet for that carb recommends. Probably 1 to 1 1/2 turns in. Keep track of that.

Bring the initial timing up to 15 degrees and plug the ported source for now. Now after warming it up, see if you can get the engine to go into gear without dying and not needed the assistance of a little pump shot. Once its fully warmed, adjust the idle mix screws. You should be able to be able to find a sweet spot (max vacuum, max rpm) between 1/4 and 2 turns out. Set it a on the rich side of that sweet spot to help keep the power when you go into Drive.

While the engine is warming up, you try to figure out the fast idle and choke positions. it may not need much of either, but it will need a little when cold if the carb is set correct for flly warmed up engine.

If you can get all that squared away, then you can figure out the other things such as
where the ported vac source is an try hooking that up and also the distributor advance total, and the vacum can adjustment. You can also experiment with a little more and a little less initial timing. Just remember that in neutral, the engine will be happy with more timing and leaner mix, but the real test is whethrer it has power when put in gear.