What is the difference between joining n activities with one activity and joining n activities with merge node and then the merge node with one activity.
As I understand the UML specification:
A merge node is a control node that brings together multiple alternate flows. It is not used to synchronize concurrent flows but to accept one among several alternate flows. All tokens offered on incoming edges are offered to the outgoing edge. There is no synchronization of flows or joining of tokens.
On the other hand,
... an action can only begin execution when it has been offered control tokens on all incoming control flows and all its input pins have been offered object tokens sufficient for their multiplicity. The action begins execution by accepting all the offers of control and object tokens allowed by input pin multiplicity.