David,
You've described it pretty well. The idle mix screws are fine adjustment to trim the bottom as needed. Otherwise, the transition and idle are pulling from the same circuit through the same restrictions. There is one difference which Tuner has taken pains to point out: The portion of the transition slot above the throttle blade is also an airbleed.
The concept of the slot as a variable air bleed is illustrated in figures 15 to 19 of Chrysler MTSC 66-5*

"The curb idle is pretty well controlled with the screw, unless of course the IFR is way too small." Or more commonly if the transition slot is over exposed. Fuel will then take the path of least resistance to the low pressure.

"The number of turns is a good indicator of whether or not the IFR and idle air bleed are correct"
I'm not so sure about that. See what Mark says. I can recall Shrinker saying it basically doesn't matter how turns out they are. His point was that as long as they are reasonably sensitive, the system is working as it should.

I don't know if a press-in IFR can be used on your blocks. Others will have to answer that. I've only done a couple and they were for Holley 4150s. I don't know why you couldn't press in a a little restrictor made of aluminim or soft brass. Mark thought I was nuts for looking for press in air bleeds for a carb, but in some cases press in is less work or the look needed. An advantage of set screws is that it makes changes faster once you have a bunch drilled. Even from McMaster-Carr, a bag of set screws is not very expensive (except perhaps shipping to you). I'd say do what works for you, but also see what others say.

There are some other threads on the Racing fuel systems forum with different places to buy taps, drills, etc.

- Matt

edit
* you may notice that the simplified explanation assumes no transistion slot exposed at idle. I think that's because they were just trying to explain the concept of how carburetors work.