For #1 if customers are assigned to groups that get a certain set of variants for each test and you have enough groups to run the full combination of tests that

For #1 if customers are assigned to groups that get a certain set of variants for each test and you have enough groups to run the full combination of tests that are active, you should be able to correlate drop off with certain tests. For #2: That's much harder. How do you handle this for larger products that you don't A/B test? I'd bet this isn't a unique issue.