I will try to answer your three questions.
Q1) Limiting the execution time for each test case.
A1) At this moment you cannot set a different run-time limit for each test case. The default test program set a run-time limit equals to the total run-time limit divided by the number of test cases.
The run-time limit can be set in the "Maximum execution resources limits" option.
Q2) How can we limit the number of evaluations/submissions?
A2) The number of evaluations or submissions cannot be limited. You can only penalize each evaluation.
Q3) how to export the highest grade among all submissions like in CMS.
A3) You cannot get the highest grade of each user, only the grade of the last submission. You can export the activity grades from the grade book of Moodle.
Notice that VPL is not developed to support programming contest.
From this post I add on the TODO list the following:
1) Add a time limit for each test case.
2) Add the highest grade as an evaluation option.