OK, I think we agree about how to implement each “Process” Node. And you seem to have enough experience to be able to implement it without too much effort.
I was suggesting that each thing that you were assuming would be a “swim lane” should instead just be a Node, occupying a horizontal stretch with a vertical panel on the left. Each such Node would have a “connection matrix” area on the right, consisting of a “Grid” Panel stretching as far as needed. That means horizontally to cover the area occupied by the “Process” nodes, and vertically to cover the items in the “Vertical” Panel on the left.
Then you would implement each “connection” as a small black “Circle” Shape that would act as a port element. You would position those circular ports at the intersection points.
It isn’t clear to me whether the top row under the “Process” nodes is itself a special Node stretched horizontally and holding arrows, or whether it’s just a fixed Part with decorations and the up and down arrows (other than the two furthest left) are decorations (labels) on the Links. I would guess the latter is the best course of action.
What are those red ellipses? Are those to delete that “parameter” or that whatever-you-want-to-call-that-horizontal-line-of-the-nodes-on-the-bottom? (You need better terms for things down there.)
I understand that the blue circles are where there might be circular connections made, but you don’t expect them to all exist simultaneously, do you? It seems that whenever the user starts drawing a new link from a Process parameter than only those available connection points should be shown in that vertical column. And perhaps at most one at a time – the one that would be made if the user did a mouse-up.
Initially you might not want to bother to implement your own custom Layout to position the Process nodes and the nodes on the bottom, but in the long run I recommend that you do so. It would be responsible for stretching those bottom nodes the correct length towards the right.
In some respects this is very similar to how the Sequence Diagram is implemented, Sequence Diagram, except it is turned around 90 degrees and the “sequence diagram activity” nodes are just simple small circular ports rather than separate nodes. So this seems simpler than the Sequence Diagram.