You bring up some good points, and it seems that you've already worked out each of your points. Good job!
For others reading, I'd like to expand on a few things --
Everything got smooth except for the updating of one language pack
This was one of the factors used when trying to determine if the version numbering should remain at "7.5" or move to "7.6". There were so many new, duplicate and unused language strings within 7.5. And there were meny new language strings that needed to be added for the new features of a major new version. Early on, putting all the pros and cons on to a list, showed that a minor version bump from 7.5.9 => 7.5.10 was not going to be feasible with all the language changes needed.
Basically, Language packs for 7.5.x will not work well if adding them to a 7.6.x series forum. There were just too many new and corrected strings added to 7.6.x.
After upgrading, I couldn't easily get into the control panel as it gave me a white screen.
I also got a white screen and had a rough time trying to figure this one out myself. But what it came down to was, my browser was caching the "index.php" file. Once I cleared my (Firefox) browser's cache, everything worked as expected. This was a totally unexpected behavior, since if the file contents had changed, the browser's cache should have been updated. I attributed this to the fact I was running a beta version of Firefox :/ since I didnt have the problem on Chrome or on any of my mobile devices when accessing the website directly.
In version 7.5.x, the index.php file points to "login.php" for the entire Dashboard.
Control Panel's Dashboard = login.php
In version 7.6.x, the index.php file now points "dashboard.php" for the entire Dashboard.
Control Panel's Dashboard = dashboard.php.EDIT:
If you've downloaded ubbthreads-7-6-0.zip any time on or after 02-17-2017 (the date of this post), a corrected archive is now in place within the Member Area. The only difference is in the link, which was corrected from index.html to dashboard.php. Most users will never run in to an cache issue. And if you've cleared your cache and can visit your dashboard, you dont need to do anything. The problem was caused by an over zealous cache. If you are using a CDN (such as CloudFlare), switch it to "Developer Mode" or turn it completely off before upgrading. Before turning it back on, clear its cache.
The only thing were the layout of the showflat/post next/previous thread buttons,
When testing the updated 7.6.x templates on about 40 of the the top 7.5 add-on styles, I came across a style settings within them that was just wrong. This might help others who come across the same issue.
General CSS Properties > .t_standard
This group should not have have its own background and colors declared. Its basically a copy of .t_inner, using only the margin and padding elements.
General CSS Properties > .t_outer
If you're having table display issues, you may have duplicated items for t_outer and t_inner, causing a "double border/margin/padding."
The border/margin/padding items within the t_outer group can be placed within the t_inner group. Before submitting your changes, click on the "Update Preview Area" button to confirm everything looks as you expect it to.