Can HDF5 be reliably written to and read from simultaneously by separate python processes?
HDF5 parallel I/O will not solve this problem.  That technology is primarily intended for performance, not for collision avoidance.
What you want is know as SWMR (single-writer/multiple-reader):

Data acquisition and computer modeling systems often need to analyze and visualize data while it is being written. It is not unusual, for example, for an application to produce results in the middle of a run that suggest some basic parameters be changed, sensors be adjusted, or the run be scrapped entirely.
To enable users to check on such systems, we have been developing a concurrent read/write file access pattern we call SWMR (pronounced swimmer). SWMR is short for single-writer/multiple-reader. SWMR functionality allows a writer process to add data to a file while multiple reader processes read from the file.

SWMR was first included in HDF5 version 1.10.0 released on 2016-03-30

Concurrent Access to HDF5 Files - Single Writer/ Multple Reader (SWMR)
The Single Writer/ Multiple Reader or SWMR feature enables users to
read data concurrently while writing it. Communications between the
processes and file locking are not required. The processes can run
on the same or on different platforms as long as they share a common
file system that is POSIX compliant.