Urghh Very frustrating. No luck yet. Changes and notes from last round of efforts below.
Cornerstone is installed. I believe on my version it was installed by default.
Added to .htaccess
php_value max_execution_time 300
max_execution_time = 140 ; Maximum execution time of each script, in seconds
max_input_time = 140 ; Maximum amount of time each script may spend parsing request data
memory_limit = 300M ; Maximum amount of memory a script may consume (25MB)
added to config.php
In my Mamp Preferences / PHP. Cache is off.