1-Not too often if only because few do the complete take apart
. Freewheels see very little bearing track wear due to a few aspects. The time the bearings are rolling there's virtually no load on them (assuming a correct amount of bearing slop is present). When they are transferring force they are stationary. Remember the smaller the rolling element the greater the effect of any change in preload.
2- This is one of the advances that the Asian manufactures brought to the cycling world (AFAIK), cog teeth with specific profiles to enhance either shifting or cross chaining. There are many different profiles that ST used over the years and between the various models and cog positions. If you look at the photo of the still assembled FW you'll see that the largest 3 cogs have tooth top with no added chamfering on the tip. If looked from behind the bike one can see these teeth have a chisel shaped tip with the sharp edge closer to the outside of the bike and the bevel going down as you go toward the spokes. This to better grab the chain during shifting (remember this design predated current indexed tooth designs by about 15+ years). The 3 smaller cogs have the added chamfer to the tooth's outward face with it removing the rear outer tooth tip. I believe this is to better allow cross chaining. Just duplicate this pair of patterns when you reassemble.
3- Very few chains are directional in their install/function. At one time some I knew were concerned about the chain's surface wear and side to side flexibility and tracked how the chain was removed then reinstalled to control this. I never bought into this and think the periodic cleaning and lubing of the chain makes a FAR larger difference in life span and function. As to whether to reuse the chain I generally consider the "stretch" wear amount as a first priority then overall visual condition next. Is the old chain still good measurement wise? Remember that the cog teeth have some amount of wear (and there's no tooth wear measurement tool that I know of that will work on this FW) so the question might be "Are the FW teeth worn enough to no longer allow the new chain to fully mesh, and thus have no skipping. The only way to truely know this for sure is to install a new chain and power away in every cog to see what happens. We do this during bike servicing routinely and we don't dispose of the old chain till we've confirmed the new chain and old cogs are agreeable with each other. Andy