Most English speakers are comfortable using the term "more likely". For example, people commonly say, "you are MORE LIKELY to have a driving accident when it is foggy". The term "MOST LIKELY" is also more commonly used. For example, people say, "you are MOST LIKELY to have an accident when there is ice on the roads" However, you do hear short expressions such as, "the likelier odds" or "the likeliest predictions". Hope this helps

They are both correct; 'more likely' is a bit more formal.

Both are correct. This will be more on your personal preference. Although, "likelier" in my opinion doesn't sound that natural as "more likely" is more common in spoken English. Example: It is likelier that I will not pass the exam. It is more likely that I will not pass the exam. John will likelier to be late due to traffic. John will be more likely to be late due to traffic. Hope this helps~!