Re: pronunciation of "Celtic".
It's a Latinate, Early Modern word, originally borrowed from French
celtique, but even there only a répechage from Classical Latin, where
celticus was indeed pronounced "kell-tick-uss" - rhymes with "puss" - with a "k" sound, just like in the ultimate source, i.e. either Ancient Greek
keltoi or a possible, actually Celtic predecessor of the Greek word. In other words, this one (word) was not in continuous use in any language throughout the Middle Ages.
Back to English, the "sell-tick" pronunciation - based on the French immediate source, as well as on the (itself French-based) general way in which “c+e” sounds in English - was actually the more usual for a long time.
This was the case until the majority of non-sporty users of the word - mainly academics - deemed it more 'authentic' to say "kell-tick". Why?
Why, in accordance to the original pronunciation in Classical Latin, Ancient Greek, as well as (assuming the name was not a Greek invention, which hasn't been ruled out) the possible Gaulish original (in Celtic languages “c+e” reads "keh" to this day), and why not, German
keltisch (German being the lingua franca of historical linguistics way back when).
As for dictionaries, most of them admit both variants as correct (but give the "kell-tick" one first):
Why There are Two Ways to Pronounce 'Celtic'