1. Do most people apply junior or senior year?
2. What are the benefits of applying junior year vs senior year (application/ acceptance wise)?
-- does it help to apply one way or the other?
3. What do people do during their gap year if they apply senior year?
4. Is it hard to miss a lot of school senior year having to travel for interviews?
1. It used to be rarer for people to apply senior year but now it's so prevalent that it could be called one of the traditional pathways. But it depends on the school. If you're applying to top schools, there are many people (I would say 50/50 or more) who have taken one or more years off before applying. Some schools have entering classes that are 1/3 straight through, 1/3 one year off, and 1/3 multiple years off. This tends to be more prevalent at top schools.
2. The best time to apply is when you are at your strongest. For some people, their junior year application is strong enough - an extra year won't add anything substantial to their application. For these people, it makes sense to apply junior year. But for many other people, their senior year application is much stronger, due to more grades, strong upward trends, better clinical experiences, graduating with honors/publications, etc. For them, it makes sense to apply senior year. So it does help to apply one way or another but the extent to which it helps is dependent on the individual and when that individual can make the strongest application. This argument applies to taking multiple years off as well.
3. Many people do research but many research assistant positions now require a two-year commitment because of lengthy training and induction periods. Others take on a variety of jobs in other industries - pharmaceuticals, consulting, finance, etc.
4. I didn't have to do this but from other people I know, it can be quite difficult if you don't plan your courses appropriately/can't plan your courses around the interview schedule. This also assumes that you will have multiple interviews (many people who apply to medical school only get one or two interviews - all you need is that one acceptance).