I would turn down the black Kelly (it is not what you are looking for and can always be requested years later if you change your mind), pass on the diamonds which you can buy at any time, and hold out for the Kelly that you really want.
If you turn down this one, your SA will probably offer you something else soon.
And your SA will also likely realize if you buy the bag for someone else and could mark you as a reseller. This is more damaging to a relationship than turning down an offer I'd think...
Totally agree. If you want a Kelly in a pop or pastel color, hold out for that. Not that you can "always" get a black K in the boutique, but it will always be produced so you can get it later. If you decide you really want the ultimate classic black K in the future, you can also get a vintage one in Box on the reseller market.
Also, OP, I hear you on the Chanel. To me, Chanel is Black, and Hermes is Color. If you already have a classic black Chanel for dressier occasions, you're set in that area for a while. Stick to your instincts and hold out for what you really want.