The whole assembly under the vanity sink is rather confusing. My model is 1999 so there are probably some differences. You are going to have to stick your head inside the cabinet and try to grope around in an attempt to figure this out. I'll agree with
@DonScribner ... the term "accessible" is nebulous in this case!
But the first thing to do is just put yourself in the mindset that you don't have to replicate the builder's pump models or even the design of the system if you don't like it. They are way out of date and all you need to do is find a suitable modern replacement.
That Shurflo Blaster looks familiar and I think it is the same as my shower sump. I have no idea what that cylinder you are holding in your hand is. If it is in any way wired in connection with the Shurflo, why not just remove it and make sure the Shurflo is wired properly to a switch? That way, you should be able to determine if the Shurflo is broken or not. Mine is on a circuit with the freshwater pump, but it is operated with a manual switch in the head. I don't have a float switch and I see no purpose for one anyway. It's too messy to take showers in there, so we never do, but even if we did, I would simply drain the sump with the manual switch.
Don't assume that the shower drainage is pumped to the bilge. Mine is pumped overboard thru the toilet raw water intake. It has to go over a ventilated loop first. The intake thru hull served 3 purposes ... 1) toilet intake (which we have eliminated by using fresh water instead now), 2) shower sump discharge, and 3) raw water intake for anchor locker wash down. Yep, it was a tangled mess to figure out.
That motor might have something to do with a washdown pump, I suppose. Oh, I just read your comment that with it removed, your Shurflo doesn't work at all. So get rid of it and wire the Shurflo so it does work. You don't need that POS if it serves no purpose. While you are messing about with the pumps, you may as well replace all the hoses. That way, you clean the entire system and learn everything about the system you need to know.