The font is in all upper case though. lol

That's easy. Have the command use the /tellraw command to with a text like this:

/tellraw @p {"text":"Do you want a diamond? ", "extra": [{"text":" Yes", "clickEvent":{"action":"run_command", "value":"/give @p diamond"}}]}

That when the specifictext is clicked will activate the command (/give @p diamond) and then give them a diamond.

Maybe you can use a minecraft driving over a "detector track" and then power a "command-block" that's placing a "redstone block" (1.7) next to another "command-block" that you choose the command :) I dont know? What is the answere?

Well you could just very simply place things like a button,lever and such that emits a redstone signal on it,or just use an external program (or 1.7) to customize the meta-data of that commandblock so that it would work,then instantly turn off.

Hmm, I have never tried that, but here are my guesses

minecart detector over a commandblock
spawner spawning a redstone falling sand entity
just a button on a command block
(In the new snapshot) use the /setblock command

If none of those are it, then you got me stumped. lol