if I buy a standalone software download version of Lightroom 6, how far of an update/upgrade path is there?
Do their paths deviate at some point where the updates only enhance the Creative Cloud version?
If that's unfortunately the case, do they consider LR 6 stand alone version to be no longer be supported with updates like lower versions?
Also, what is the highest 6.xx version/update that can used as a standalone?
The updates ALREADY deviate from the Standalone to the CC versions since 6.0 There is the NEW Dehaze sliders and adjustment brushes and a few other updates and probably more in the future.
You can upgrade to LR6 from any previous version.
LR6 will, and has been recieving updates, but only bug fixes and support for new cameras. No new features - the new features in LR CC will probably be implemented in LR7, if there will ever be an LR7.
The current version is 6.2.1, and it's buggy, and has a dumbed down import dialog. Adobe says that version 6.3 will bring back the old import dialog.
If you buy LR6, you should immediately after installing roll back to the last stable version - 6,1,1.