Rather than change the values in bug_report(_advanced)_page.php, I added defaults for them. Diffs follow:

diff -r mantis/bug_report_advanced_page.php original_src/mantisbt/bug_report_advanced_page.php
45,46c45,46
< $f_severity = gpc_get_int( 'severity', config_get( 'default_new_bug_severity' ) );
< $f_priority = gpc_get_int( 'priority', config_get( 'default_new_bug_priority' ) );

  $f_severity                             = gpc_get_int( 'severity', 0 );
  $f_priority                             = gpc_get_int( 'priority', NORMAL );

diff -r mantis/bug_report_page.php original_src/mantisbt/bug_report_page.php
37,38c37,38
< $f_severity = gpc_get_int( 'severity', config_get( 'default_new_bug_severity' ) );
< $f_priority = gpc_get_int( 'priority', config_get( 'default_new_bug_priority' ) );

  $f_severity                             = gpc_get_int( 'severity', 0 );
  $f_priority                             = gpc_get_int( 'priority', NORMAL );

diff -r mantis/config_defaults_inc.php original_src/mantisbt/config_defaults_inc.php
567,570d566
<
< # bug defaults
< $g_default_new_bug_severity = 0; // defaults to none selected
< $g_default_new_bug_priority = NORMAL;