I think this is one of the cases where passion matters. Theoretically, I think your initial thoughts would work structurally, so long as no Signpost #3 scenes from any of the throughlines happen before you finish out all Signpost #2 scenes.
That said: Whoa, this sounds amazingly powerful, deep, and strong. Thus, structurally, this is probably an excellent interpretation of the theory. To paraphrase Melanie (a co-creator of Dramatica), though, when it's battles between passion and structure, passion wins.
An aside: Melanie mentioned in an interview at some point that Dramatica doesn't make writing easier, but harder. What I think she meant by that is that the theory is designed to help find weak or missing scenes in the story and provide thematic ideas about how to correct them. However, it won't say "Do this thing here", but instead, it will say, "Think of something along these lines; that's the kind of scene that will improve this," or "It might be better to put this there, based on what you're trying to say."
Another side note: Generally, when referring to Journeys in Dramatica, the expected reference is to that area between the Signposts, which you can see when you pull up the Plot Progression window. In the Windows version, that's the window with the four by seven grid. A Journey follows from the space between Signpost #1 and Signpost #2, and is more experiential than structural.
The Plot Sequence Report, when viewed structurally, breaks each Signpost down into four miniature, "thematic signposts". It's not necessarily a set of scenes that happens between the larger Signposts, though it may turn out that way when you write. So, using the term Journey with them can be confusing.