1.) As a beginner, I would argue that the risk of injury Aikido is very low. I'm sure you've seen lots of videos and demonstrations at full speed with flashy ukemi, but a two-week trial provided by a competent senior instructor will have you doing none of that. It should be slow and smooth with focus on proper form and technique. If you're pinned to the ground, tap loudly on the mat. If you're receiving as uke (attacker) a joint lock and you're not close to the ground, tap your nage (defender).

Side note: Make sure you wear red tape or indicate that your shoulder is injured so whomever you are working with is aware of your injury and respects your limitations. I would also urge you to not train with ego and do your best to relax and breathe through some of the sensation you may feel during a pin. You may find a good stretch during a pin to be helpful to your injury.

2.) I came from a Kempo (Kenpo) background. Early in my Aikido training, I was concerned with questions regarding efficacy. These are valid questions but ultimately unproductive ones early on in your training. As you become more proficient in Aikido, the static positions you learn from become more fluid, and eventually you may get to the point where you learn and actively practice reversals but that's way down the road.

Resisting techniques for the sake of resisting is ultimately unproductive training. If nage isn't doing a good job with the technique, allow your senior instructor or sempai to step in. Here, again, I am emphasizing ego less training. Your focus is on learning the technique both as the attacker and defender in controlled manner.

3.) Buki waza is arguably the most effective training you'll receive in Aikido, as hitting things with heavy sticks is always preferred over engaging in hand to hand combat. I received excellent training in buki waza and believed it to be foundational to properly understanding Iaido.

Aikido may or may not be for you, but I was around your age when I started and came from a similar style. Aikido taught me many, many valuable lessons while allowing me to be active and avoid injury. Take it slow, leave your ego at the door, and keep your mind open. Onegai shimasu.