Checking the lobe lift would be my first thing I would do. If good, then you can search other problems. I think it is no use wasting a lot of time checking all the other possibilities when it sure looks like lobe problems.
Never heard of 400 series engines that opened up to get any lube from crank case. All comes down from the top with different variations of nozzles that feed a stream of oil down the push rod.
Like I said before, cam failures did occur with side breathers but only after a whole lot of hours where as the top breather, before they added better oiling had premature failure rates.
Just to add a little more information. The original version used a small v shaped slot in the rocker arm support bracket to provide oil leakage down to camshaft. Probably never even notice it if you were not looking for it. Then they added nozzles, different sizes to different rocker arm supports. Then put nozzles on all supports.
So, sometimes that went smooth and worked fine. But, sometimes the oil supply was not sufficient to those nozzles. They are fed from a groove in the block under the rear camshaft bushing. That groove and the passage through head can get slugged up or just too small in the first place.
With all the cavitation of sleeves on original versions of those engines, before we got into adding a filter with coolant additive or just adding to the coolant, a lot of engines were tore down. At that time it was easy and normal to pull the lifters to check for wear . A whole lot of lifters were replaced at those times which again extended the life of the camshaft. Severe pitting was what usually showed up and not actual wear patterns.
You just never know what you will run into but some things to think about.