Will multiple-post just this one to address the band 6 issue separately (but may merge my own posts later).
Now, on the other hand, your chances of a band 6 will be affected by your trials to a different degree. As you should know by now, your final mark is dependent 50% on your internal results, and 50% on your external final exam. Within your internal exams, your trials are usually weighted the most; somewhere between 35% to 60% typically. Will your trials impact on your final results? To quite some extent, certainly yes,
That being said, it's without a doubt not the most. The trials still fall under the umbrella of internal assessment, and unlike the external mark your internals have to undergo moderation and what not, so your actual internal mark might not even mean anything. Your final exam performance still has the greatest say on your final mark as it is never moderated, AND makes up 50% of the final mark just by itself.
You can't simply calculate how much your trials or any other internal assessment will impact on your final mark. You can only have a rough idea.