I'm an LPN. I'm going to be dong lab research at my undergrad. I would love to do clinical research at some point and have considered becoming an RN for that reason in the past. I'm sure it depends on your state and the kind of research you want to do, but generally the PI would be a doctor, PA or NP, but RN's can play important roles and what exactly those are would somewhat depend on level of education. An ASN most likely isn't going to be running the show but an MSN might be able to conduct their own research. For an LPN we would be taking care of more patient care type things, and documentation. Medical assistants would not have a significant research role - not that they aren't valuable members of the healthcare team, but their jobs are usually kind of like a cross between a secretary and a CNA (very simplified explanation so I hope not to offend any MA's.)
If you want to do clinical research your best bet out of the careers listed is RN. However I am concerned about what information you have on bridge programs. LPN programs range in length from about 12 to 18 months. LPN to RN programs usually let you skip about a year of nursing school, but sometimes it is less, and many will also require other prerequisites, such as college level anatomy. You may not be able to transition directly into the program, and in some places (like my state) these programs are decreasing in number.
What I'm getting at here is that it would most likely take you at least a little longer than 20 months. Also keep in mind this would let you transition from LPN to an Associate degree in nursing. You would most likely need a bachelor's in nursing to be competitive for research positions.
Would you mind giving me a little more info as to your goals? Have you been accepted into an LPN program? If not you might be better off just entering directly into an RN program. Why were you considering LPN to RN?