Firstly, using the phrase "which I expect to be accepted immediately" when you don't already have a prior agreement to reference with this condition is just automatically going to get anyone's back up. This right here is a prime example of how NOT to start diplo with another kd. Right off the bat, it paints you as an entitled individual who seems to think they are the one controlling the flow of the conflict.
Second, stating you are going to send a cf and then failing to do so for a significant period of time is poor diplo. It portrays you as someone who cannot be relied on to follow through with their word. If you are going to delay sending the cf, don't tell them you will send one. Again, it just pisses people off.
Thirdly, you have stated you don't want war. That automatically puts you in a submissive position in negotiations. The other kd knows they can keep hitting and hitting if you don't submit safe in the knowledge the button will not be used, and since they patched gains throttling for in-range kds, they can and will end up hurting you significantly if that happens. If you want to be seen as a kd that won't roll over, don't open diplo with that.
Lastly, before sending a cf, always open discussion with the province you believe to be online to confirm they are actually around to accept the cf. If they say they will accept it straight away and then delay, withdraw the cf and explain why. Resubmit the cf when you have come to an understanding about what happened, or when you feel you need a cf at all costs.
Diplo is all about two-way communication and understanding. Sometimes you have to fudge some details, sometimes you omit things, that is part of the game. But the way you handled this incident says a lot of negative things about you as a negotiator and kd leader.