Lightroom 6 does not include Lightroom mobile. There is a trial period when Lightroom 6 is first installed, but that goes away. What I don't understand is how you can purchase and install Lightroom 6 under your Adobe ID when you are already subscribed to Lightroom CC. As I understand it, it seems you are going to confuse your purchase. But I might be wrong.
That may be why initially I could not find the option to buy, so I chatted to Sales Support who gave me the direct link to purchase.
Is there a cancellation period if just ordered it tonight?
I have no idea. You'll have to chat with sales support again. I'm just a user like yourself.
Please note that LR sync with LR Mobile can only happen from One computer and from One LR Catalog on that One computer to multiple Mobile devices. LR Mobile is not for syncing 2 installs of any version of LR across 2 computers.
If you try to sync up with LR Mobile on 2 different computers, even using the same exact catalog file, it will drop, disconnect, LR mobile from the first computer and wipe out all the synced images. Then if you go back to that first computer and re-sync to LR Mobile it will wipe out what you did on the second computer.