You're on the right track conceptually, but took a slight detour getting there.
The descriptions of these systems can be somewhat confusing, having to do with the definition of 'above' and
'below', when we're talking about negative pressures (aka 'vacuum'). Technically, 5 in/Hg vacuum is less negative
than 10 in/Hg of vacuum. Kinda like how 4.10 gears are 'lower' than 3.55s...
Operationally, it is as you say. At idle and light throttle openings, the TAD system is under control of EGR vacuum.
But the TAD solenoid is energized in this mode. Both TAB and TAD solenoids are normally closed at rest, while the
TAB vacuum switch is normally open at rest (atmospheric pressure).
<edit>
After further review, the '85 TAD solenoid is normally open, and the '85 TAD vacuum switch is normally closed.
Apologies for the misinformation...
</edit>
At idle, EGR vacuum is weak, so the diverter valve sends air upstream to the exhaust ports. At light throttle openings,
EGR vacuum is strong, and the air is sent downstream between the beds in the main catalyst. This reduces exhaust
temperatures in the upstream light-off cats, while also maximizing NOx reduction.
With heavier throttle openings, where manifold vacuum is 10 in/Hg or 'less', the vacuum switch opens, de-energizing
the TAD solenoid. This blocks EGR vacuum, and vents the diverter valve, sending air upstream into the exhaust ports,
until manifold vacuum drops low enough that the bypass valve opens and thermactor air is dumped to atmosphere.
The TAD system works essentially the same for '83-'85 4V engines, the main difference being the '85 vacuum switch
transfers at 6 in/Hg, vs 10 in/Hg.
As an additional safeguard, the TAB solenoid is under control of the WOT switch on '83 and '84 4V engines. The TAB
solenoid is de-energized at WOT, blocking manifold vacuum to the bypass valve. (For '85 4V engines, the TAB solenoid
is controlled by the TSAD module, which does the same thing, but will also shut off the air supply during extended idle
periods.)
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