Also, i'd be interested to know if removing the call to project_cache_all at the end of core.php has any noticeable effect on the mantis installation with your workload.
After the previous failures, I have comment the project_cache_all in core.php

Now atleast I get the xdebug error
( ! ) Fatal error: Allowed memory size of 16777216 bytes exhausted (tried to allocate 131072 bytes) in /home/aghaffar/mantisbt/core/project_hierarchy_api.php on line 146

continuing on further disections now