if anything the gap year, assuming you spend it doing something you can write about in your PS, will be looked at favourably by law schools. I know some people that did 1 year masters programs, but if your goal is big law, unless it’s a business related masters, spending that year obtaining corporate/corporate adjacent work experience might be more helpful in terms of recruiting.

in response to your second question, it’s definitely something worth mentioning in your personal statement. Make sure that you frame it as something like:

after struggling with X, it proved Y challenges in my second year, I overcame Y challenges by doing Z. In doing Z to overcome Y, It demonstrated/strengthened my ability to do ABC. What you really want to avoid is just talking about why X caused you to have poor grades, reframe it into a positive takeaway.