Q: How can I increase the service interval?

A: I think it will be very difficult to have long maintenance intervals.
In any case let me dispell a few myths: engine valve clearance rarely needs adjusting as long as valves and seats are of good material. Replacement parts can be sub-standard and a lot of problems in the past came from poor quality valves.
Despite all your attempts the crankshaft bearings will probably remain the prime target for any dirt floating in the oil (remember they are bi-metal bearings, so-called "sacrificial" because they were designed to be "worn" while sparing the crankshaft from damage). It is unlikely your filter will catch it all immediately even if you increase its size (have you noted how large the Fulvia filter already is?) because of its upright position at the back of the engine.
Another area of concern are the camshaft bearings and journals: because of the design they can trap dirt with obvious bad consequences for engine longevity.
Thirdly, the lifetime of the timing chain will critically depend on the oil. The only solution is to use clean oil rather than hoping that something will clean the oil for you. Fulvia engines are not prone to lack of lubrication (like other Lancias) but they were designed with a short interval maintenance schedule.
Finally, even if you manage to enormously extend the engine service intervals, you will still have to look after things like oil in the steering box (don't they all leak?), in the gearbox, grease points, etc.
- Andrea -