The colleges that require the mid-year report usually have a deadline for them on their websites.
When I did mine, my Careers Advisor just updated on what I've been up to and how I've been doing since the initial application.
I didn't submit my Final report as the system was taken down before then. Theoretically, they could rescind their offer if you bomb your A Levels but I've never heard of that happening (possibly because I've never heard of anyone bombing that hard). My uni didn't ask for anything from me after I committed. Others may vary.
There is nothing against shooting the colleges you are applying a quick email asking about what to do in specific scenarios, by the way. They will be more than happy to help you.