On your first question...
This is pretty much spot on, but I think it's even a bit broader: it's all about the influence itself. (How it changes is definitely important, but that's specifically shown in the Benchmark and the Signposts).
Here's the thing: the IC Throughline represents an alternate perspective, basically the opposing perspective to the MC's perspective on their personal issues. It's that perspective and its influence on the MC that's important, not the IC character himself.
Because of that, any IC story point has a kind of AND/OR built into it. Any IC story point can be about the conflict the IC themselves is experiencing (as long as it somehow -- in any way -- brings influence to bear on the MC). AND/OR it can be about how the IC is influencing the MC.
So, an IC Signpost of the Past might mean the IC is experiencing conflict from the Past (say, a past affair threatens him), and somehow this impacts on the MC. Or it could be the IC is dredging up something from the MC's past, causing conflict for the MC. (In either case, the IC might be doing it on purpose, or might not even be aware. It's only the influence that matters.)
This is how Wilson, the ball from Castaway, can be the influence character. It's also why you can have group ICs, hand-offs, etc. Because it's all about the perspective and its influence.