I have a 1996 Johnson 15 hp 4 cycle (model J15FREDR), and two nice new spark
plugs that really want to take up residence in the motor. The top plug is
easy to get to with a spark plug socket. But the bottom plug's within the
"shell" of the motor, and it looks to me like there's no way to get a
straight shot with any sort of wrench. I'm really not in the mood to crack a
plug.

The left side of the shell is held on by star drive fasteners - no big deal,
except that the fuel tank attaches to its connector on that side of the
shell. What sort of nightmares await, in terms of getting that connector
loose so the shell will come off?

If it's a level 8 through 10 nightmare, it's off to the dealer.
Otherwise...maybe.....in the driveway.