TD 5 has a different design clutch operating fork, a cast item similar to the V8. Thus unlikely you have punched a hole through it. Might be failure of seals in slave cylinder, if it was the master you would have an indication from the tell tale leaking into your footwell.
The gearbox can be removed from below on a Defender, but is heavy, very heavy. It is also possible to split TB from main box whilst they are still in the vehicle. Drain it first because at least one of the bolts fastening them together goes into the oil reservoir of the TB. You will probably need to remove the tunnel anyway, in order to get to the various linkages/cables which operate hi/lo, diff lock etc
How long the job will take depends upon the sort of facilities and equipment you have available to you. In essence it is possible for one person to do it over a weekend.
Personally, I would do it from above and endure the removal of seat box etc. But I don't have any way of lifting the vehicle, neither do I have a pit. I have done it both ways, but when I did it from below, the engine was out if the vehicle so I didn't have to worry about lining everything up to get the mainshaft into the clutch.
Get some M10 stud bar. Then if you do separate TB from main box in the truck and put it back the same way, you can screw lengths of stud into some of the threads for the bolts fastening them together, thus taking some of the weight and preventing the TB from "rolling" while you refit it.
Bon chance!