Just a couple points about one of your points...

When you say that Accushift was designed for primitive chains and so had to overshift on the downshift, you are essentially correct. But the reason for the overshift approach wasn't so much because of the chain, it was because Shimano's tight patents prevented Suntour from copying Shimano's superior hyperglide cog tooth profile and centeron self-centering upper/guide derailleur pulley. Shimano's successful indexing relies on the cog teeth aggressively grabbing the chain on the downshift, with the centeron pulley automatically centering itself to maintain tooth/chain alignment and keep the drivetrain quiet. Suntour had to rush indexing into production and were unable to find analogs to Shimano's cog tooth/pulley without infringing on Shimano's patents. The overshifting on the downshifts was the only way they could make it work.

The click in Shimano's lever just had to get the chain close enough to the cog centerline for the twist-teeth to grab the chain, while Suntour had to bully the chain onto the flat teeth by overshifting and praying the teeth, with enough grinding, would pickup the chain. Accushift upshifts rely on precisely dumping the chain onto the cog, praying the cog centerline will be centered well enough to accept the dumped chain. Consequently, Accushift was very intolerant of off-spec components and frame dimensions, and setup/adjustment was finicky even when everything was dead-nuts on-spec. SIS was much more forgiving, was always easier to setup, and seldom needed adjustment.

Another complication was Suntour claiming that rear derailleur shifting geometry was consistent across the entire product line, requiring only one indexing disc/pattern for Std-6 and only one for Ultra-7. This was not true, there were at least 3, maybe 4 different geometries, which should have required dedicated indexing discs. Consequently, some lever/rder combinations were more marginal than others.

One upshot of all this is that though a modern chain may improve Accushift performance, Accushift's problems are much deeper than just the chain. Flat teeth combined with an overly-long rear dropout "drop" dimension (axle-center to rder pivot-center), crappy cable/casing and poorly-matched lever/rder can still potentially cause problems. I'm not at all discounting your own extensive experience working on this stuff, I'm just cautious from having spent many, many hours manning the Suntour dealer line, trying to help righteously-pixxed dealers, at a time when we had hundreds of thousands of under-performing drivetrains in the market. The first wave of 6spd drivetrains were horrific enough for any sane dealer to never order another Suntour-equipped bike, and that's pretty much what happened.

But if I can ever drum up enough enthusiasm to install an indexed Suntour drivetrain on one of my bikes, though I'm not sure that day will ever come, I will certainly try using a modern chain.