the way that link is, it's the way that it's always been (or at least last 6+ years)... in v5.3, it used to look like...
so you can see in 6 years, it's hardly changed...
there was a feature request to add the back button to the top of the page, but it's 3 years old, got 2 votes and is now archived.... before that feedback thread, I can't recall anyone else asking about it - probably because you can do easily do it in the template.
the point being that WHMCS won't (practically can't) agree to every request - they just provide ways for the user to make changes yourself, e.g templates, hooks, addons, css, feeds etc.
not easily! viewinvoice doesn't use a header or footer, so you can't use the output hooks to inject JS into the template... i'm not saying it couldn't be done, but it's far simpler to just edit the template. 🙂
I know Aug/Sept was a crazy period with updates seemingly being released every week, but updates won't occur as often for a while.... supposedly there is going to be a 7.9 before the end of the year, but I personally wouldn't expect that to be more than v7.8.4 (e.g v7.8.3 + hotfixes) + Hosting Renewals... but time till tell... once that's out of the way, you might not see v8 until early Summer 2020 the way things are going.