Word-Usage – Correct Term: ‘Back-End’, ‘Back End’ or ‘Backend’?

Two words are joined by a hyphen when the collocation occurs in a non-standard context. For instance, we speak of the front end (space, no hyphen) of a car when this acts a noun phrase:

The front end of my Chrysler was dented.
My Chrysler was hit on the front end.

But when we use front end as an attributive, we hyphenate it:

My Chrysler suffered a front-end collision.

This lets the reader understand that we are speaking of a collision involving the front end rather than an "end collision" (whatever that might be - it has no obvious meaning) at the front.

In the same way, the phrasal verb is always speed up (space, no hyphen) when we use it as a verb:

Whenever we're behind schedule we speed up.

But if we want to use speed up as a noun, we hyphenate it.

We were behind schedule, so the foreman ordered a speed-up.

Two words are compounded - stuck together with neither a hyphen nor a space - when the collocation acquires a distinct sense, or the phrase becomes so common that it is felt to be a single word rather than two separate words. For instance, a dead line or dead-line was originally a line around a military prison which a prisoner could be shot for crossing; but when the phrase came to be applied metaphorically to the point by which a project must be completed, and became very common in the sense, it turned into the single word deadline.

Tenses always apply to verbs, so to see where to apply it, you need to figure out which part of the compound (or hyphenated) word is the verb.

"Rollback" is a compound word, consisting of the verb "roll" and the preposition "back", as Peter indicated. As such, "rolling" is what you are doing, and "back" indicates where you're rolling (as opposed to rolling forward).

"Rollbacked" would imply that "back" is the verb, as in "to support". Unless you're supporting the edit with a cinnamon roll or used said cinnamon roll to move it back, this is not correct.

A converse compound word would be "backup" -- here, "back" is the verb and "up" is the preposition (see the etymology of 'backup' in its non-compounded form). You haven't "backupped" the files, you've "backed up" the files.

With regards to double-click, "double" is the adverb that modifies the verb "click", so "double-clicked" is the correct form. I can see this possibly being confusing because it could be seen that you're "doubling" the click (or past tense, you "doubled" the click); however, when you tell someone to single click, you just say "click the button", and thus it becomes more obvious that click is the verb here.