I'm a third generation harness maker in England my granddad dealt wholly with work gear this equipment spends its time either being worked hard on a very large horse or laying on the stable floor (wasn't the beautiful smell of leather you was greeted with when you entered his workshop most of his work being repairs in this type of work) this harness was hand stitched with hand spun linen thread (most called it hemp but its linen and was cheaper than ready spun thread) waxed with black wax and I've seen sets of harness around 100 YEARs old still in use no sign of thread rotting also seen items where the entire top and bottom of the stitches have completely worn away and it's still holding together as far as I'm concerned that's tried and tested so its linen for me but silk was supposed to be best never used silk don't know anyone that has. I do use braided polyester ready waxed on some jobs like sewing box keepers where their isn't enough room to use needles wouldn't use it to sew anything that will be exposed to sun light though I've seen it rot nylon in acouple of years, polyester is probably same, it do's hold up well to hand sewing. As to the subject of twist I asked my dad this many years ago and he said that thread twist only makes a difference to machines, before the advent of machine's twist wasn't an issue have known him to be wrong on occasion though. I'm right handed and use z twist same as I use on the machine a lot of hand stitching I do is continuing on from machine stitching so needs to match, on occasion that I make a thread I don't pay attention to the direction of twist and I've never noticed a problem I don't slant my stitches though that's an American thing we like ours neat and in line and English is regarded as best;), don't lock every stitch by passing the threads over each other I just do this at the ends or maybe it only shows up on long runs, Ill have to hand stitch a pair of traces and see.