The only question that really matters is this; are you going to finish your degree with a 2:1 or higher overall? If the answer is 'yes', all of the other issues become largely irrelevant. It may be that you get a question in interview as to why your law degree took four years rather than three, in which case you can provide a simple answer. But otherwise providing you tick the box of getting a 2:1 or higher in your degree, you are unlikely to encounter any issues arising from falling short of a 2:1 in your first year.
If the answer is 'no', the situation becomes a little more complicated. You may well be able to point towards extenuating circumstances, but the harsh reality is that whoever is reading your pupillage application will also have read countless others that boast a 2:1 without the need for extenuating circumstances. Your 2:2 therefore becomes something of an issue. Not, I will stress, as much of an issue as those that have a 2:2 without extenuating circumstances, but an issue nonetheless. If you end up with a 2:2 it may well be an idea to do a Masters to demonstrate the 'true' level of your intellectual ability, though in fairness your A-Level grades do that to some degree as well. But otherwise your application will have to stand out to the extent that it mitigates the '2:2' on your application form, and that is not an easy thing to do.
As regards your mini pupillage application, it may be that your 2:2 in your first year contributed to you not getting accepted. It may be that it played no part at all and the Chambers in question just had far too many applications. There's plenty of time for you to secure mini pupillages, and indeed you should look at approaching Chambers that don't have formal application processes and/or are likely to deal with a lower volume of applications (generally smaller/regional sets) as well as applying to larger sets.