80 Mph is quite high revs for a 1.3, especially if it is a 5 speed, I have a 2014 Vauxhall Combo1.3MJ which is basically the same as yours, in 5th gear 50-60 mph is where it seems happiest, The 1.6 van I have which has the six speed gearbox is much faster, but doesn't have the low speed grunt.
What is described isn't actually a compression test, more a quick "blow by" test, taking the oil filler cap off when idling will show if excessive piston/cylinder bore wear by the amount of fumes blowing out of the filler, it is a guide but needs to be based on experience as for instance a worn petrol engine blow by is much less than a good diesel engine. For accurate compression test on a diesel as suitable high pressure gauge would need to be fitted to the heater or injector plug holes with an adaptor. Then if readings are low I then use a leak test involving another piece of equipment that puts compressed air into the engine on each individual cylinder and can show where the pressure loss is, piston/valves/head gasket etc.
At the beginning when you mentioned taking the intake pipe off, I suggested, "When you say it is leaking oil from the intake pipe, do you mean the side where the air cleaner is or the side nearest to the inter cooler?" This would have told you if it was a turbo fault or an engine fault where oil is forced out of the breather before the turbo and then sucked in through the "good" turbo.
Re the white smoke, I have had a 1.6 MJ that had a faulty turbo where the bearings had gone and you could feel excessive movement, this allowed the oil lubrication to go past the worn bearing and burn in the engine. Even with the new turbo it took a long time to get all the oil out of the intercooler and pipework so that it would stop smoking.
