Yes, it's a valid two letter scrabble word.
No, 'xo' is not a valid word in Scrabble. 'xi, xu, ax, and ox' are some of the valid 2 letter words in Scrabble containing 'x'. 'Xo' is valid only if you are playing Clabbers, a variation of Scrabble where and anagram of a proper word is allowed.
No - The only valid two letter words using X in scrabble (interestingly you can create them with each of the vowels) are: AX EX XI OX XU
No. Qu is not a valid Scrabble word.
Yes. Speculate is a valid Scrabble word.
Nope. Indew is not a valid Scrabble word.
Yes, "keyword" is a valid Scrabble word.
Yes, "scramble" is a valid Scrabble word.
Yes, "Scrabble" is considered a valid word in the game of Scrabble.
Yes. Ex is a valid Scrabble word.
No. Oz is not a valid Scrabble word.
Yes, "lez" is a valid word in the game of Scrabble.