Hi Nigel
1. Good time to change to flexible hoses across the De Dion tube. Always difficult to remove the pipe from the caliper as the nut tends to corrode to the pipe and twists it as it's removed. you may get enough wiggle room by releasing the P clips that hold the pipes to the De Dion.
2. The drive shaft will just pull out once the De Dion ear is unbolted.DON'T touch the toothed ring. It should be locked, moving this will change the pre-load etc on the bearings which carry the diff inside.
3. No need to drain the oil.The oil level is below the bottom edge of the seal. That is unless you plan to change it. If so an oil pump can be used through the filling plug to suck it out eg Brass oil pump
4. As far as I know the oil seal will be a standard size - check with SPC Components, Redline etc they will know for sure
John