http://en.wikipedia.org/wiki/Hepburn_Romaji
> Double (or "geminate") consonants are marked by doubling the consonant following the っ, except for sh→ssh, ch→tch, ts→tts..
In practice, if you search Google on words other than "ecchi", (such as "acchi" vs. atchi", "kocchi" vs. "kotchi") you find that both are in use. "cchi" is actually w?puro and not Hepburn: http://en.wikipedia.org/wiki/W?puro_r?maji
> W?puro-style representation of long vowels is popular with non-Japanese fans of anime and other aspects of Japanese culture who are unfamiliar with (or unable to type) more formal romanization methods, as well as people who like the fact that it more closely reflects the Japanese kana orthography.
> [...] The Hepburn spelling tchi for っち may be rejected, and cchi must be used instead.
That is to say, your IME doesn't know correct romanization.
> Hentai is not only a cursing word.
> Hentai: transformation, metamorphosis, abnormality, pervert.
> Metamorphosis seems to be a valid word, maybe used by entomologists.
The "metamorphosis" usage is far less common, and probably not well-known to the general populance. Do you know that "orphan" in English can mean a single line of text from a paragraph that has been split to another page from the rest of the paragraph? A lot of fairly common words have specialized meanings that are only used in jargon. It has very little to do with common usage, and isn't an argument in this particular discussion.
> So who am I to believe? Those proclaimed (male?) Japanese language experts who claim that hentai is a horrible, horrible term that no one has the right to use, or plain evidence from countless anime and games?
I think you'll notice (see >>4 and >>13) and so on that the people saying it's weird are Japanese speakers, not "proclaimed Japanese language experts". It's not that it's a horribly rude word, it's just that it has strong negative connotations and using it in a neutral (or positive, like on a porn site advertising it has LOTS AND LOTS OF HENTAI!) way is very, very strange and creates a whole lot of cognitive dissonance.