How to decode Playfair when only a small part of key is available?
Playfair key grids are often generated by filling in the letters of a key phrase, then completing the grid with the remaining unused letters of the alphabet.
It looks to me like the partial grid you provided has been created in this way. If so, you can fill in several more letters straight away (assuming there is no J):
? A ? D ?             ? A ? D ?
? ? ? B C             ? ? ? B C
G ? ? K L     --&gt;     G H I K L
N Q ? ? ?             N Q ? ? ?
V ? ? ? Z             V W X Y Z

Furthermore, the three cells between Q and V are most likely to be RST, RSU, RTU, or STU. For each of those four possibilities, there are now only six cells remaining to be filled. With a computer, it wouldn't take long to test every possible combination (4×6! = 2880). You could probably even solve this with pen and paper by attempting a partial decryption with the parts of the key you already have, then working back from the plaintext to figure out the others.
I think it's safe to assume that TXT2 is the Playfair encrypted message. It has an even number of characters, no digits or punctuation, and no letter J. Also, when split into pairs, there is no pair in which the same character appears twice (something that cannot happen in Playfair encryption).