Revision Tracking Graph
If you insist that the edges in the complex merge equation don't appear twice (eg. that the + you use is actually a disjunctive union), and you only subtract sets that are superset of one another (*), I think the assertion you want to prove (4) is actually false. Imagine RTG with 5 vertices and 6 edges:
V = { 0, a, b, c, d }
E = { (0,a), (0,b), (a,c), (a,d), (b,c), (b,d) }

now, if To = c and F = d: C(c) contains already all the edges except (b,d) and (c,d), So Sum (Fi-Bi) has to be exactly those edges. Since these edges are only contained in C(d), they must be (not necessarily both) contained in summand d-B for some B. But that is not possible, because whatever B we choose, we add also edges that are already in C(To).
(*) otherwise the assertion (2) is actually false: Result = To + (From - To) satisfies the simple merge equation for all From and To (even if it looks bizarre).
Long story short: You gave inadequate or incomplete definitions.

Q: Why do we have to state definitions rigorously if the proofs are usually trivial if the definitions are right?
A: The definitions are almost always wrong.