Which games fail due to ECS or 1Mb Chip RAM that work with OCS and 512k Chip (with or without 512k fast)? Double Dragon 2 is the only one I know of, where the A500+ issues weren't caused by Kickstart 2 (and even that was fixed pretty quickly, long before the Plus launched)/

Double Dragon II and Eliminator. They were poorly coded and didn't address Agnus registers properly, which failed on the new Agnus. Not a memory issue.
Impossible Mission II and I believe a few others didn't mind the new Agnus, but expected RAM at $C00000.

What do you mean, long before the Plus was launched? Tons of games failed on the A500+ because of the KS, to the point that many people returned it to the shop and demanded a regular 1.3 A500 instead. This was only alleviated when relokick was published... In 1993 !

I used Zkick, which I think came out before Relokick? Used it to load KS 2.0 on my Amiga with 1.3 and I think this was before I got an A1200 in late 1992 but could be wrong.

I meant that the issue with Double Dragon 2 was fixed before the Plus was released. Having got an ordinary A500 not long before the Plus was released, and having friends who got Plusesthat Christmas, I'm fully aware of the issues with that. Not as bad as the initial panic made out, but bad enough. Still, some of the failing games got officially fixed fairly quickly (eg the budget releases of Lotus 1 and SWIV).

I never understood why, if bedroom coders could come up with Relokick (or ZKick, which doesn't ring a bell) and solve maybe 99% of compatibility issues using software, why Commodore couldn't've done the same much sooner.

I'd always assumed the 1Mb of Chip RAM was in the same place as the 512k of chip RAM previously, rather than being moved (which, forgive my non-technical brain, is how I'm reading the comment about Impossible Mission 2)

Ah, ok, I thought you meant that issues with KS 2.04 were fixed long before the Plus release, which I would find surprising. You were only talking about the issues with the new Agnus.
Actually, it is the trapdoor slowram that was "moved" (to $C00000). When the extra 512K is chipram, it is contiguous to the first 512K, at $800000.