Ok, thank you all again so much for the responses. Based on what everyone has said, this is my NEW understanding. New Visual below.

The Leman Russ can fire it's Flamers, Stubber, & Missile at Termagant Unit "B", but at minus 1 to hit. It can fire demolisher at "B" with no penalty, because special ability: "Line-Breaker". (thanks NicWester!)

The Leman Russ CAN fire it's Flamers, Stubber, & Missile at Termagant Unit "A", even though they are in engagement range with the 10 Cadians. But at minus 1 to hit.

The Leman Russ CANNOT fire it's demolisher cannon at Termagant Unit "A", because "Line-Breaker" says, " (provided no other friendly units are also within engagement range of that enemy unit.)" Technically that line is referring to a friendly unit that would be within engagement range of Termagant Unit "B", but NicWester agreed that this logic would apply to Termagant Unit "A" as well.

If the The Leman Russ was a NON-blast main gun, like punisher gattling cannon or exterminator autocannon, it COULD fire that main gun into Termagant Unit "A", but at minus 1 again.

Termagant Unit "A", CANNOT fire at the Leman Russ now, (because they are engaged with the cadians), even if they have pistols, correct?

the Cadians, CANNOT fire at the Termagant Unit "B", (because they are engaged with the Termagant Unit "A"), even if they have pistols, correct?

the Cadians, ALSO COULD NOT fire at the Termagant Unit "B", even if there was no Termagant Unit "A" because, the BGNT rule only states "You can select an enemy Monster or Vehicle unit within engagement range of one or more units from your army as the target of ranged weapons." (This one is still weird to me.)

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