Damaged connection in one of the plugs? Although I would have expected you to find that when you checked for continuity.

If you jack a front and a rear wheel up so they are both clear of the ground, with the gearbox in Neutral you should be able to spin the wheels. That will show you if it is the parking brake and maybe highlight anything else. Propshaft UJs shouldn't line up, they should be offset. Workshop manual for the Classic shows it but P38 doesn't as normally there is one spline missing on the sliding joint so it can only be assembled correctly. However, as it is a CVC it may be that they used earlier props.

No, the spray on glue dries as soon as it hits the surface so nothing will stick to it other than something else with another layer of the glue on it.

The two fir tree fasteners can't be seen until you remove the D pillar trims, the ones at the very back. The pillars are A down the side of the windscreen, B, the ones at the rear of the front door and front of the rear door, C, behind the rear doors and D at the very back. With them out it's only the door and tailgate rubbers that are holding it up (and the sunroof surround if you have a sunroof). If you recline the front seats and remove the rear seat headrests (or drop the rear seats down, it can be tilted and pulled out the back.

If the height sensor tests OK and swapping it didn't cause the fault to move to the other side, it isn't that. If the wiring shows continuity (and no short to ground) then it isn't that, so that does only leave the ECM. Maybe that is why it was converted to springs in the first place?

Every gasket you need to remove and if I was doing it for myself, rather that stretch bolts I'd fit a set of ARP head studs. The stretch bolts come in various grades of quality, too soft and they stretch too much (or break) and don't hold the head down firmly, too little and you can stress the threads in the block. The studs have the coarse thread where they go into the block but a much finer thread on the top where the nut goes so you have more control of the torque. Do them up to 65-70 ft/lbs (in 3 stages, 40, 55, 65) and the job is done, not cheap though. Don't try putting the studs into the block then fitting the heads, put one stud in (centre top is a good one) to guide the head down then once it is on, screw the studs in hand tight. Head bolts (and studs) need a 16mm socket and I've found a 1/2" drive socket for the smaller headed spark plugs is a perfect fit and usually 6 point too.

Your memories will come flooding back, the A series is a pushrod 4 pot and so is the V8, it's just two of them.

To allow the tailgate to open, you need to ground the Green/Red wire from the RH front latch, either at the latch or at the connector behind the RH rear panel in the boot.

RHD drivers side latch has 3 microswitches, as does the passenger side although only 2 of them are used, so you'll probably get one microswitch more than you need. You don't have a key lock on the RH side so the keyswitch won't be required.

All suppliers are showing it as subject to suppliers stock, which would be Land Rover. Unfortunately LR themselves are showing it as out of stock (https://parts.jaguarlandroverclassic.com/fqj103240-latch-assembly-front-door.html). Tailgate locking is supplied by a ground from the RH front door latch so if that door is unlocked the tailgate is too. I don't see why you couldn't fit a RH RHD latch though.

What's the air gap on the compressor clutch like? Mine was well worn and too wide (it should be between 16 and 30 thou) and the feed from the HEVAC goes through the notorious multiway connector in the RH footwell (as well as via the Trinary switch), so a combination of a wide air gap and a bit of resistance in the connector and it wouldn't kick in. I tried putting a relay in place so the coil would be energised from the HEVAC and switch full battery volts but that won't work on an early HEVAC. It looks at the amount of current being drawn and if it sees a lower than normally expected amount, it assumes a problem and doesn't try to energise the relay any more.

No, not coded, you just need to get the correct one. You'll have the 4 wheel TC rather than 2 wheel on a GEMS.

It looks daunting at first glance but once the inlet manifold is off it's just a pair or 4 cylinder pushrod engines sitting side by side. It's a weekends work in reality if you've never done it before, a day the second time. If you've got the overhaul section in RAVE, then it gives you step by step instructions with lots of pictures, torque settings, order for doing the bolts up, etc. It's not difficult at all, I'd rather do head gaskets than wheel hubs in all honesty, there's nothing there to seize up. Island do a complete head gasket set, with all the gaskets you will need (and quite a few that you won't need) or you can buy head gaskets, valley gasket, valley end rubbers and rocker cover gaskets separately if you prefer.

Dodgy wiring somewhere then.....

again.

Acetal, Delrin, Nylon, wood, whatever you have handy or can get hold of easily enough. As long as it is strong enough that it won't be crushed by the weight of the car. With the Access height ones I found that they almost completely disappeared inside the bumpstops making getting them out again difficult. So I drilled and tapped an M6 thread in one end so I can screw a bolt in to pull them out with. Also marked big arrows so I always put them in the correct way up (so the threaded hole was visible).

Yes, it isn't difficult, just a bit awkward laying underneath the car manhandling the weight of it. You obviously got a decent one. I bought a DA1065 tool and found I still couldn't get it onto the nuts so use a 3/8" drive 14mm socket on a wobble end extension. Still not easy but gets the job done.

Try swapping them as it will at least move the operating point to a different part of the track. You may find that you then get errors with it on High instead.

You've got an ABS ECU problem. Speedo is driven from pulses from the ABS ECU so that, coupled with odd ABS and TC errors suggests ECU.

Was it sitting level when you got those readings? A difference between 108 and 144 from side to side is far too great. Stored values look a bit odd too, you need to make yourself a set of blocks and do it properly.

Valley gasket is pretty straightforward and not difficult. See what the other 6 core plugs look like (3 on each side of the block). If they are starting to look crusty then chances are the ones at the back of the block are too. If they look good, then they should all be much the same.

Screen will only fail MoT if it is cracked. Insurance here will pay for a stonechip to be repaired as that is cheaper than waiting for it to turn into a crack and need replacement, so they may well cover taking it out and rebonding.