How do I hold up Miriam's arm to get past the door?

The default KB+M setting for aim is the right mouse button. You have to hold it down, and then you can move the mouse around to aim her hand. In testing with changing the aim bind to a key, I still had to use the mouse to actually do the aiming while the key was held down. Couldn't find any way to remap that behavior. It might be a bit funky to get it to work with just the keyboard, since the aim function was likely made with a standard two-stick controller in mind(kind of the same way Owl Boy is a bit of a mess to play without sticks due to the intended aiming setup).

You're going to need some additional software for that. Something like xpadder or joy2key, which allows you to bind mouse movement to keys. I'm not sure which the best choice would be in this case, since I've never needed to work around the lack of a mouse. You'll need to do a bit of research. Is something wrong with the trackpad on your MacBook?