1) All of the West park stuff should have Poser and DS installers - which product are you not seeing a Poser installer for? If an item doesn’t say it’s for DS only (or for another application entirely) then yes, missing Poser files would be a problem with the content or the product page and should be reported.
2) Newer products with a readme in the documentation centre should list updates, otherwise no there isn’t a way of knowing short of resetting. The forthcoming install manager is billed as fixing that with update notices.
3) That would be an error - report it, but you can work around it by refusing to uninstall or by simply not installing the uninstaller.
4) Lighting is generally the trick - though with things like metals, if they use real reflections, it’s also necessary to have things in the scene for them to reflect.
5) Unimesh add-ons just add new morphs, they should have no impact on the way materials apply.
6) Choice of target application never makes a difference to which files are installed, as far as I know, just to how the installer looks for possible install locations.