I think, if I were you, I would just give the lower dose until I get the dental (unless preshots are too low ). He may run a little high at preshots, but he won't dip down too low. After the dental, you can look at adjusting it down further. And if then, you are still getting the wild swings, come on and ask for advice from UK people. I do think we have a few people in the UK who are able to get insulin other than Canninsulin.
I think your instincts have been spot on since the first day you came back on. And certainly, more logical than your vet's dosing advice.
I can't see your spreadsheet. Did you "publish it"? The URL on your signature has key in the URL. (Keep that one in your favorites on your computer) That usually is the one you use to add numbers to. The one we can see but can't alter is the one with pub in the URL.
B. Publishing your SS, aka Attaching to signature
1) In a new browser tab, open up your google doc
2) Click on the tab on the upper right "Share"
3) click on "Publish as a webpage"
4) Click the box that says "automatically re-publish when changes are made" (see pic below)
5) Copy the http link generated for you in the box at the bottom (see pic below) (use control C or command C to copy the link from the circled part in the picture above)