BPMN11 — Events in subprocesses

The spec is unclear on when the events are enabled, ie, "listening" for
the event.

For example, suppose an independent process has a start event (either
message, timer, or rule), and this process is used by another (as a
subprocess, for example), as shown below.

P1:

A ---> P2 ----> B

P2:

Start
Event ---> Other activities
(Message,
Timer,
or Rule)

What will happen if A is executing in P1, and the start event for P2
occurs before A is finished?

If P2 is intended to be called only by other processes (it isn't "top
level"). Is there a way to prevent if from being triggered
independently of its callers?

see above