Thanks Robin.
The mkdir confirmed that the install was being prevented from creating directories. It was more than just write permissions though. So here’s the scoop for anyone else encountering the problem. PHP scripts run by default with user ‘nobody’. As the files are owned by my username, PHP is unable to write to them or modify them. The solution is a script wrapper allowing the files to run under a real username. I used SuEXEC so that instead of using the Apache default, PHP files are executed using CGI.
So installation works, files are uploadable through the browser, files are parsed and thumbnails are generated. All good news. The permalink to the image generated works but, for now, the image is not displayed on the public view of the site 🙁 The only limitation of using CGI for PHP I am aware of is the inability to send custom headers to the browser but can’t imagine why custom headers would be used here.