The Shure A81WS are overkill indoors, but yeah - you've got it about right. Before hitting a show, play around with different mic configs using your A27M bar and shockmounts with mics in them - that way you won't have to sort out how to set up DIN, or ORTF, or XY, or point-at-the-stacks at the venue.
I forget the inner diameter of the Shure A53M shockmounts - just make sure your 480s will fit (I think they will).
Yup, the clamp to which you linked will work well - and no need to adapt threads, it comes with a 5/8-27 male thread which will suit your A27M 5/8-27 female thread just fine.
FWIW, I like my Signal Flex MH-10 better than the KM238 - I don't think SF makes it, it's the
same one available over at Cascade Media and a bunch of other retailers. Search around and you'll find a better price than $18. It allows you to clamp to stands easily as well as chairs, tables, railings, etc. - more flexible than the KM238, IMO.