Unfortunately mkenuk has vanished from the forum, and he is possibly the only person who would be very interested in the unraveling of this issue. However, he did draw attention to a quirk that ought to be sorted out.
To summarize,
smoothe is not accepted in Chi, but
smoothes is accepted - it's not regarded as a "plural" (i.e. an inflection made by adding
S) and, what's more, it's classed as a common word. This is consistent with one dictionary at least, the American Heritage. This dictionary has the verb
smooth written without an
e, and with no alternate spelling, but the verb inflections are given as
smoothed,
smoothing,
smoothes.
However, other dictionaries have different ideas. As Mike pointed out, some of the Oxford dictionaries give
smoothe as an option for the spelling of the verb. Others make no mention of
smoothe or
smoothes. Merriam-Webster spells the base form of the verb as
smooth, with no alternatives, but allows
smoothes as an alternative to
smooths.
Garner's Modern English Usage describes
smoothe as a misspelling.
In contemporary usage, some people do write
smoothes, but
smooths appears much more often. A few people also write
smoothe, but this is often when the word is used as an adjective, so it's a moot point whether it is a spelling variant or a mistake.
Some examples: From the British paper the
Express, in January this year:
That tech enables something called Action Mode, which smoothes out your video footage as you jog or sprint...
And from Scottish paper the
Herald in August last year:
The fund could be underwritten by the Government, or a willing financial institution, and repaid over a 10 to 15-year period to smoothe out the costs.
I think the best I can do is add
smoothe as a rare word, and tag
smoothes as a plural.