I recommend you take a look at the CI/CD Automation support package: https://www.mathworks.com/products/ci-cd-automation.html
One of its main capabilities is performing incremental builds, which only run tasks with outdated results to reduce your build time. This is especially helpful if you are commiting your models and code frequently so that the changed artifacts are only a fraction of the total test suite.
On your second question, to generate a single set of results for the different test runs, all you need to do is load all of the results onto the test manager. If they are all loaded you'll be able to save a unified report.
If you are collating the results from multiple pipeline runs, you'll need to export the results as a .mldatx file at the end of each run and store them somewhere so that you can import them later onto the test manager to generate the unified report.
