There should be spreader plate on the back (? the gearbox side) of the crossmember. Genuine crossmembers also had reinforcing tubes to prevent the crossmember getting squeezed. Some of the non genuine replacements don't have this feature.
Also, if you cut any vegetation you must leave it, it belongs to the Land Owner. After moving a tree you cannot chop it up and take it home as firewood.
It might be easier to look at a Puma 6 speed, as that at least uses a version of the traditional Transfer Box.
Doubt it's worth the time and effort though.
You're right that if the wheels are locking up, better brakes will probably make things worse, although they may give more control enabling more sensitive application.
It sounds as if your problem is more tyre related, as well as possibly a bit of driving style. A Defender is not a GTI and...
Are you sure it's the drain plug, and not the wading plug in the bellhousing?
If you have been driving round without gearbox oil I think you would have known some time ago!
Depends where the 2.5 came from. If it was a LR, it should be fine, if it was a Sherpa, then the timing cover is not sealed.
You'l need to know which before you buy a belt, as they are different.
I believe the current officials wanted to stand down having done the job for a number of years, but no-one wanted to take over.
They had a formal meeting a couple of weeks ago to decide the future of the club, but I don't know the outcome.
If their site is down it doesn't look promising though.
What sort of off roading are you doing that a Sport won't do on standard tyres?
As Crazymac says, 19s will give you a higher profile tyre, which tends to be better off road, as well as far less prone to punctures.
Not sure of the exact year, but it was a bit later that the arrival of the Discovery 3, and coincided with the figment of the "Jaguar" V8 in the Petrols, and about a year later the TDV8 (replacing the TD6) in the Diesels.