Apply some combinatorial testing technique

Candidly there are quite many moving parts, and there are few alternatives to address all of those:

• Systematic processing of those aiming to automate the checks for regression
  • The integration test forms a baseline for this
  • Pros: High coverage, easy to spot which part stopped working
  • Cons: Requires efforts to do this, and if there will be some significant changes, the maintenance of the tests could require extra efforts
• Exploratory testing
  • So far luck has been with us, and the lurking bugs have been spotted
  • Pros: Quick focus on the actual testing, no need for lots of test implementation
  • Cons: Requires quite good domain knowledge, and at need buddy for debugging incidents (keyboard external error, implementation error, design error, undocumented feature etc). Manual regression testing with multiple release candidates become laborious, and there is no time to explore further
• Combinatorial testing techniques
  • See e.g. ISTQB material for suitable approaches (few alternatives)
  • Pros: Quite wide coverage with reasonable efforts. For limited testing time, this might be good.
  • Cons: Requires some planning. Requires support from coders to understand what are orthogonal features