As mentionned by @pnuts in the comments, you will find left/right arrows at the top right of Gmail (next to the Settings gear, as shown below).
As a side note, you may also use keyboard shortcuts to go faster:
press j to go to the next message
press k to go to the previous message
AFAIK there is no way to use the back button to easily get tot he previous position in a doc.
If you are trying to do something like jump from one spot to another and back, what you can try is something like a Back to top link.
For example: if you have a table of contents at the top of a doc that jumps to headers below, you can insert a Table of Contents ...
You should "hack" the Google Form as the required feature is not built-in.
It's worth to say that old forms are easier to hack than the new forms, this because the old basically use "standard" html elements while the new use Google Clousure, an undocumented library.
Regarding "hacking" the olf forms see my answer to How can I make a minimal, unstyled HTML ...
Google Sites doesn't include a feature to set a page order that is site wide. The order is set on each component, if this is available.
The list page gadget doesn't include a way to set a custom order for pages so pages are ordered alphabetically. The straightforward solution is to add the links manually to the page so we will be able to add them in order ...
I don't know the ordering sequence for Google Sites but the basic principle I would suggest is to build in a lot of 'redundancy' from the start, but for convenience in a compact form. Say you have three pages to start with, then something like:
E5 M5 V5
and if 26 pages you might use the entire alphabet. The numbers would allow you to slot in five extra ...