> “Inbetween” is never written as one word. If you have seen it written in this way before, it is a simple typo or misspelling. You should not use it in this way because it is not grammatically correct as the noun phrase or the adjective form. https://grammarhow.com/in-between-in-between-or-inbetween/
Matthew 7:3 "And why beholdest thou the mote that is in thy brother's eye, but considerest not the beam that is in thine own eye?"
Also called a "back-formation". FWIF I don't think the existence of corrupted words automatically justifies more corruptions nor does the fact that it is a corruption automatically invalidate it. When language among a group evolves, everyone speaking that language is affected, which is why written language reads pretty differently looking back every 50 years or so, in both formal and informal writing. Therefore language changes should have buy-in from all users.
And linguists think it would be a bad idea to have one:
https://archive.nytimes.com/opinionator.blogs.nytimes.com/20...
If you intend for people to click the link, then you might just as well delete all the prose before it.