^ Thanks, Maurice. Good info.
About the motive for creating Chekov, I think the Davy Jones idea is the most reasonable. The Monkees originally aired on NBC beginning in the fall of 1966--a year before the introduction of Chekov in TOS season 2, so it seems Monkee-mania was so potent, NBC (and/or GR) desired that same kind of feel pushed into TOS (after watching the tone of season 1), in a way that would fit. He was not singing, or getting into romance of the week, but just his presence (character age factor, wig, etc.) had the potential to draw in younger, female viewers.
I guess we should be thankful Chekov wasn't given singing scenes like early Uhura was. Regarding the Monkees connection, here's a box item from Shatner's 1993 TV Guide article: