I don't see that your rear main questions where addressed. You shouldn't have to drain the oil to change the seal. Make sure the new seal comes with the gasket for the plate the seal rides in. When you get the plate off, take a picture of how far in the hole in the plate that the seal is installed. It will be important if your crankshaft is scored from the seal lip - if the seal is all the way in the plate, install the new seal a little bit further out - there should be enough room in the plate. This will result in the new seal missing the score on the crank. Make sure you support the plate when installing the new seal or you'll warp the plate.
When I removed my transmission, I used a tow strap from the front of the engine to the rear sway bar to keep it from moving. You can also use a bottle jack under the engine to accomplish this.
I don't recall a need for specialty tools, maybe a large socket for the shaft nuts. I do recall having a hard time trying to find anaerobic sealant for sealing the transmission halves upon assembly but this is back when I lived in a tiny upstate NY town 15 years ago-it may be readily available in your local store. Make sure you order everything you need all in one shot, nothing like realizing you need something and then waiting another week for parts to arrive. Other things you should replace while you're at it is the bellows 103131 and seal 103049 along with the seals for the drive axles if those are leaking. I again recommend a new bushing kit for the shifting linkage and a new crossgate cable. I thought my old crossgate cable was good until I got the new one and compared. I even tried to grease the old one with no luck.
Biggest thing I can tell you when you are putting the transmission back together: do not go by the DeLorean manual for installing the differential, you will install it backwards. Be sure that you DO NOT INSTALL THE DIFFERENTIAL BACKWARDS! You'll be heartbroken when you go to pull your car out of garage and can't because you have 5 reverse gears and 1 forward. I made this mistake- nothing like having to drain your new transmission fluid, remove the transmission, crack it back open, etc.