What testing strategy and tools would you use for this case?

So I have just started to work on a testing project, we are trying to see if the unit tests implemented for an open source software has the followings:

1. Do we have enough test coverage? say 60% or above
2. How good is our test cases, meaning if we make mutant, does it fail or not?
3. Should we implement a new test set?

Here are some general information about this software:

• I- It is a bioinformatics tool implemented in our lab, but the author is no longer working here.
• II- This software is implemented in c++ and python
• III- It includes two types of test (regression and unit tests). Unit tests implemented by cppunit.

So I was wondering what would you guys do? more specifically:

• Where would you start? would you measure test coverage first and then move to mutation testing?
• What tools would you use to measure test coverage?
• What environment(IDE) would you use to run the current test suite?
• What application would you use to generate mutants on current test cases?
• Imagine you decided that current tests are not enough, would you keep using CppUnit or you would switch to a new tool like google-test etc?