I would like to know if the sump is under pressure
If, by the main seal, you mean the crankshaft seal, yes, you can get an oil leak from there. Usually the oil will drip from the bellhouse/gearbox join .. and blow along the underside of the car as you drive. If you've put a new crankshaft seal in the problem should have been cured.
However, it seems as though you're hinting at another problem?
If the sump is being pressurised that points to an engine problem - broken piston ring or worn engine, though a broken piston ring usually results in a trail of blue smoke when you drive along ..
With an engine that is starting to show signs of age and wear, the upcoming piston compresses both fuel/air mix - and of course exhaust gases - which, in a worn engine can find their way down past the piston and consequently pressurise the sump.
That build up of pressure in the sump will seek a way to get to atmosphere in any way it can - usually removing the oil cap when the engine is running may reveal a 'chuffing' as fumes are escaping in time with the beat of the engine. Removing the dipstick with the engine running may reveal fumes being blown out at varying degrees of pressure.
I once owned an Escort 1.8 diesel. The first signs of a worn engine were the 2 rubber washers underneath the rocker cover nuts being forced outwards - because of the crankcase pressure.
If your crankcase is being pressurised because of a worn engine the oil in the system will be forced to atmosphere wherever it can - seals and gaskets.
I don't own a Fiat with a diesel engine - however, I'm aware that some owners have had problems with oil finding it's way to atmosphere via the injector seals.
The problem here - I believe that the Fiat 1.9 has a precombustion chamber - is that a build up of carbon and goo in the precombustion chambers raises the compression and results in exhaust gas/oily goo being forced to atmosphere past the injector seals.
Whether a build up in the precombustion chamber would cause the crankcase to become pressurised, I don't honestly know.
Just as a point of interest, my Escort diesel had 200,000 on the clock when the symptoms of crankcase pressure began to show. Ford engines aren't 'high mileage' units. I swapped the engine for a secondhand unit.
Fiat engines - I don't know about that. Maybe somebody else can tell you what the life-mileage of your vehicle is.