The implementation should probably be as simple as the following, which will cover all cases and possibilities of routing:
Each splitter has a Dest 1 and Dest 2 with corresponding slider (0-100%) setting how much of the signal should be passed to each destination. By default these are set to Dest: none and Amount: 0%. Feedback should probably be forbidden (splitters can only route forwards in the chain).
An example of a flowchart and what order the devices would be put in a device chain (several solutions are possible, some less optimal). As seen, a splitter with one of its destinations muted would function as an “end bracket”.