Re: Can't drop lower unit
I have an '02 version of that engine in 2 stroke so I looked in my service manual just to be sure of what I am about to say. Hopefully the 4 stroke is identical in this area.
As Chris said, there are 5 stud/bolts. There are also 2 dowel alignment pins, one near the very front and the other near the very rear of the lower unit but they are between the midsection and lower unit and cannot be seen externally. For that reason don't expect to be able to twist it to get it off. They possibly could be part of your problem even though they are probably SS and would highly resist corrosion; much better than the alum housing and both would have to be corroded to keep you from seeing any light on either end.
The drive shaft is installed with a thin layer of grease at the powerhead and I think the receiver in the crank is bronze. With a SS shaft, one would not expect that to be your problem. Even if it were you should be able to get some light between the two either in the front or rear. Shifter has some room to wiggle so that isn't the problem. The water tube is copper so that isn't the problem.
Apparently, as the guys have said, your alum mid section is corroded and stuck to your alum lower unit so you should be able to follow their directions and get it free without hurting anything.
After soaking, you might get a 2x4 and a hammer and put some smacks on the LU right at the base of the housing. Don't get out on the wings of the anti vent plate and/or splash guard as they can/will break off or bend.
HTH
Mark