1) If you'd like a dedicated Leadership space that doesn't include any pre-leadership dates or hours (like when you had a general membership role with the organization earlier), then you'd pick Leadership-Not Listed Elsewhere, provided you delegated tasks to others for whose efforts you had responsibility. You have the option of splitting out the non-leadership time into a second space (tagged Extracurricular), or alternatively, referring to the earlier years as part of a backstory in the narrative of the Leadership space.
If you had a year or two with the organization prior to the leadership and you want to include all those hours and dates in one space, or if the leadership role means you do all the work and don't delegate tasks to others, then you'd use Extracurricular. But you can still call attention to the leadership role through the name you give the space, as well as through your description.
2) Research experiences for which you got class credit can be listed in the Activities section, provided they involved original hypothesis-based research and you feel the experience makes you a more attractive med school candidate. Campus posters and presentations don't add much cachet as they are often required regardless of content and don't undergo a peer-review process. It looks to me like the independent research experience might be worth adding.
3) Don't fluff up your application. Keep it succinct. The average spaces filled are 9-10. If the two scholarships weren't based on merit in college, don't include them on a med school application. You could probably get all four of those recognitions into one space. Is Deans List worth including, when your GPA speaks for itself? You decide.