Here's my proposal: horizontal connections should only be used when the upper object has only one outlet, and the lower object has only one inlet. Otherwise there will be always be ambiguity.
With your example of all outlets pointing to the same inlet, there is still ambiguity as to whether or not all the outlets are actually connected. That's fine if your patching style is to connect them first, and only line them up after. But if someone is debugs a patch they made that includes your abstraction, they would have to click and move objects to be certain all the objects are actually connected.
There is one way to be a little less conservative, but it requires a usage agreement: if a horizontal connection travels from left to right, it must come from the right-most outlet of the upper object and must connect to the left-most inlet of the lower object. Reverse if the connection travels from right to left. The problem is there's no good way to tell if a patch adheres to the agreement.
Matt