Boston to Mansfield with a bike

I am a new student at Boston University, and I am pretty unfamiliar with the Commuter Rail. I've used it a couple of times but never with a bike. I want to take the Providence Line on the 20th to visit a friend in the Mansfield area; however, she doesn't live directly in Mansfield, so I want to bring my bike to get to her place since she doesn't have a car to pick me up and Uber is expensive. However, I'm a little concerned about the logistics and would appreciate some advice from people who have experience with the Providence Line and/or bikes on the MBTA

I understand that Providence Line trains allow bikes on weekends, but that there are limited bike spaces. I was thinking about boarding at Back Bay since it's closer to BU, but would I be better off going to South Station to improve my chances of getting a bike spot?

I'm also a little concerned about Back Bay in general since it's a sunken platform. I'm not sure how large the elevators are or whether my bike will fit. Are there ramps or another way to get a bike in and out of the station, or would South Station be significantly easier?

If I do go to South Station, how early should I arrive before the train to make sure I get a bike spot?

I'm also concerned about crowded trains, especially on the return trip. I currently plan to take train 5868 so that I can get back to Boston before dark, but the timetable says it is often a full train. Even though bikes are allowed, should I consider taking an earlier/later train because of the limited bike capacity? How often do people actually encounter the bike area being full?

I also understand that Mansfield only has mini-high-level boarding. Since Back Bay and South Station have full-level boarding, how should I position myself when boarding (eg front, back, or middle of the train) to make sure I'm in a car that can use the mini-high platform at Mansfield? I'd really like to avoid having to take the bike down the stairs.

For the bi-level cars used on the Providence Line, I've read that bikes are stored near the doors. Is there seating near the bike area, or will I have to stand with my bike for the entire journey?

Finally, what happens if the designated bike area is already full when I board? Am I simply out of luck and have to wait for the next train?

Any other tips for someone taking a bike on commuter rail for the first time would also be greatly appreciated. Thanks!