Forever and for ever

There are lots of expressions, mostly with quantifiers (all, every etc.), that can be written as one word or two. Generally, it makes a difference to the meaning:

Every day: daily (it rained every day for a week)
Everyday: common, unexceptional (it was an everyday occurrence)
All right: everything correct
Alright: agreement
Any one: free choice of one out of all of these (you can choose any one ice cream)
Anyone: existential or universal pronoun (anyone could understand that)

And so on.

But I got to thinking about forever/for ever. Most of the time, it doesn't seem to make a difference which one you use:

It was taking for ever to finish the essay.
It was taking forever to chop all the potatoes for the stew. 

It's sort of surprising that, given that these other expressions have different meanings, that it would just be free variation. For isn't a quantifier, but I don't know that that should make a difference. I checked my copy of Fowler's and found the following:

1. It's two words when the meaning is 'for all future time' or 'in perpetuity' (he said he would love her for ever).
2. It's one word when the meaning is 'continually, persistently, always' (the children are forever asking about more pocket money).
3. It doesn't matter when the meaning is 'for a long time' (as above). 

I still think you could get away with forever even in the first one, though perhaps not for ever in the second. Almost free variation, then.