I'm sure this is right, but I also believe it can be difficult to prevent confusion when doing this if one is in the early process of learning Dramatica or storyforming their story. Though, it is nice to keep in mind for more complicated stories.
Absolutely. In my own WIP, the player (character) that embodies the Influence Character for the first three Signposts of the IC Throughline is completely and utterly onscreen. However, at the end of the third Signpost, he secrets his way out from the village where he and the MC were staying. This signifies the start of his Change (the MC is the Steadfast perspective), but it's not the end of his influence:
The final Signpost is Learning, and the final influence on the MC comes from the MC's Learning that the IC player left so he could try to figure out what the MC seems to have that the IC doesn't.* In essence, it's the final piece that the MC needs to strengthen his foothold, but it comes from what is left behind, not from anything that could be called a "character" per se. At least, that is my current outline for it, and I kind of hope to keep it that way, but we'll see what happens when I get to the actual writing. (This is kind of the opposite of what you describe as it's influence from absence at the end, but it holds the same idea. There isn't a player on the board to represent an IC, but the empty space is enough to influence the MC. Basically, so long as the influence can be connected, it should work.**)
*I'm not sure exactly how this is going to work, yet. I was thinking a note, originally, but I think I want to come up with something a bit more unique in the storytelling. Maybe something that uses the magic system or the like...
**And, as @Greg said, it might not even need to be connect. I have an ongoing experimental story where I have four separate tales, with characters that never meet, built using Dramatica. There's some kind of ineffable connection among the four tales, though. Something strange and fluid, but not quite right to a Linear mind about how "not connected, yet connected" the tales are.
Unfortunately, I don't have answers to the other questions, so I'm looking forward to hearing from the crowd.