I don't like that 2X4 technique on steep roofs either, the only time we ever used them was above the work area to prevent bundles from sliding down the roof. Even experienced roofers can have weak ankles, I've seen one too many take a roll to lower staging with a sprained or dislocated ankle from slipping off the 2X4s. I constantly harped, "Feet pointing down or up roof never gable to gable."
Back in the early 80s I worked for a man that came from England, he was farmed to a roofer as a boy and learned the trade, all past and current forms from the ground up, I learned to do slate from Herb; the application is similar to the old asbestos roof and sidewall shingle tiles. The man was an encyclopedia of roofing info, metal and soldering, roll, rubber, hot tar, resaturant, emulsion, slate.
I'm guessing the roofers you saw using the 2X4 nailed over existing shingles used roofing cement to plug the holes. Lift the course, plug the hole, lower the course press it in then plug the outer hole. An older version is to slide a strip of copper coated with roofing cement between the courses and press that in. An earlier form was to coat the copper then apply a layer of asphalt based roofing fabric over the copper then hit it with roofing cement. The best way is to leave that course out till you're done and destaging to apply the course.
Most roofing equipment today isn't geared for steep pitched roofs, steeples or mansards, Like you one has to construct platforms to create level areas for materials and rest spots. I still have a couple adjustable pipe stage roofing platforms I used once in the late 90s to set pipe stage on a lower roof to stage up to a higher gable so I could remove the old molding, sidewall and replicate the molding to replace.
Older steeplejacks used bosun's chairs to work the roof today it's rappelling gear, as long as you can still get up to the eye and hook in.
Work smart not hard!
Never bite the hand that looks dirty