I cannot figure out how to get a wrench onto the bolt that attaches
the top of the shift shaft to the shift linkage. From the left side of
the engine, I can barely see it for all the fuel hoses, lower shroud
pieces, etc. Even if I ever get a socket on the bolt and undo it, I've
even more worried about ever getting it reattached later

Use a 1/4" drive long extension with of course a 1/4" drive universal socket,
size 3/8" to loosen and tighten that bolt. (Ratchet, extension, socket tools)

To remove and install that bolt, use a long flexible mechanics tool (finger
like grip tool).

When installing the lower unit, have the shift rod in reverse. To line up the
shift rod, you'll need to insert a large screwdriver (or something) under the
lower hood pan to hold/pry the shift rod slightly forward to keep the holes
lined up.

Be careful not to turn the shift rod as the measurement is critical.

Joe (30+ Years With OMC)