4
You could do this using a Process Builder (the downside is this uses more CPU so you could later hit CPU limit issues if you have a lot of other processing going on when contacts are updated).
If you stick with this trigger-based approach note that you are doing far more iteration than needed and there's no need to query the account instances.
I recommend ...
2
The first idea that came to me, is to add a technical field on account (flag, default = false).
Extend your CSV with the new flag, set for all = true
Update accounts
Extract contacts with condition where Account.MyFlag__c = true
update contacts
Delete flag (since not more useful)
1
A good pattern to follow when writing a testmethod is the Given-When-Then pattern
// Given a mock Account
// When Account Updated
// Then Verify Contact created
then fill in the details
// Given a mock Account
Account mockAccount = new Account(...);
insert mockAccount
// When Account Updated
Test.startTest();
update mockAccount;
Test.stopTest();
// ...
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