Indeed you can use the tasks under the “Reflected” category for all such sort of things like get/set properties, call functions etc (blackboard variables can of course be used there, yes 🙂 ).
Regarding the UI button click condition, I just tried this and it seems to work fine. Please note that for Unity UI interactions to work, there needs to be a Unity object in the scene called “EventSytem”. This is a Unity object you can create from the Unity menu “GameObject/UI/Event System”. Can you confirm that it exists in the scene please?
FSMs and Behaviour Trees are quite different in how they work and as such each has its own set of nodes. In an FSM Action State all actions can be set to run in sequence (this is the default). In Behaviour Trees there is a Sequencer node to be possible to Sequence whole branches rather than just actions, which is required for the flexibility a Behaviour Tree needs. Are you saying that you not seeing the “Sequencer” node in the Behaviour Trees node browser please?