REF: simplify operating-columnwise dispatch
After this we have one usage of split_and_operate left and 2 places where we can _almost_ use `@maybe_split`.  In both cases I'm still trying to untangle things.
> rebase after we merged the other one, but have some questions here

rebasd + green.  questions? updated to re-write and re-use split_and_operate rebase, but need to relook rebased, green ex two issues on master being addressed elsewhere ok improves on existing (tangled code), splitting block ops not very clear where its happening, but I am sure future PRs can help address.