Alex,
IMO your first problem is to figure out what is going on with the carbs.
You should not need to change the needle in order to get them as lean as you
like at idle. If that is not the case, then you have a leak inside the
carbs (leaking fuel into the throat) or badly worn jets/needles, or some
other problem.
Worn jets are a particularly nasty problem, since they change the mixture
calibration curve. If you adjust the mixture at idle to compensate for the
wear, then the mixture will go lean under cruise conditions, which can lead
to overheating and potentially severe engine damage. I believe worn jets
were the "root cause" for my Dad's TR3A swallowing a valve many years ago.
Then you need to address whatever is wrong with #4. Sounds like you may
already have a burned valve, possibly due to the carb problem. Last time I
had a cylinder that only did 60 psi cold, the valve head looked like someone
had taken a slice of pie out of it.
To answer some of your questions, yes, it's fairly easy to drop the pan with
the engine in the car. No need to remove the clutch slave, just back off
the nuts on the support rod so you can remove the bolt through the pan and
swing the rod out of the way. Lower the pan slowly, so you can maneuver it
around the oil pump screen without breaking the screen. (However, the
screen may be already broken.)
If you suspect a leaking head gasket, it might be worth doing a check for
combustion gases in the coolant before starting to disassemble. I have seen
several cases where the gasket was not obviously "blown", but was leaking
enough to cause problems.
Note that the pan bolts are not all the same length (or at least shouldn't
be). The short one goes in the front, the two long ones go at the left rear
(one for the road draft tube brace, the other for the clutch slave brace).
On my TR3, I found that some DPO had changed almost all of them to the wrong
length, and the threads in the front hole were damaged as a result.
-- Randall
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