1. I use a good quality plumbing silicone. Clean up extra with denatured alcohol
2. Tighten it till the nut won't turn anymore. Just like a basket strainer. If its cast iron you won't hurt the drain outlet.
3 i have done this just as you describe. worked ok but it was a short taipiece (about 8") and it was abs which I think glues together easier then pvc so I'd have some concerns. Still prolly easier then lifting over the pipe. If your ptrap is below slab it will help if it is on firm soil. Take careful measurements when laying out the trap.
4. Wet the gasket with water before pushing it over the pipe. This helps it slide into place. You may need to pound the gasket into final position. A straight vertical drain pipe helps (see #3) You can leave the pipe a little long and cut off the excess after setting the gasket by using an inside cutter.
Be sure you don't pry under the tub/sink apron. It will chip the finish. (trust me on this) Try not to drag it on a rough surface either this will also chip it
Best if you have helper for this one. good luck