Is single-word "inbetween" becoming more acceptable? How far can it go?

I get the distinct feeling that "inbetween" occurs increasingly often as a single word, but I'm not at all clear on why it's used more in some contexts than others.

What I can is see that in Google Books, "are inbetween" occurs far less often than "are in between", whereas "the inbetween" occurs more often than "the in between". What's going on?

I had not previously been aware of seeing it printed other than as two words, but the practice seems not to be particularly new. The OED records the hyphenated noun in-between as meaning ‘(a) An interval. (b) A person who intervenes.’ The first citation is dated 1815:

He's fallen in love with Lady Naglefort, because she's an in-between.

It’s followed a year later in Jane Austen’s ‘Emma’ by:

Busy . . . talking and listening, and forming all these schemes in the in-betweens.

As an adjective meaning ‘placed between’, it occurs first, once again hyphenated, in 1898:

White or pale-coloured silk, with an in-between layer of chiffon.