Kelvin cannot cancel abilities with spacebar or any other keybind.

[thechamploo]

Cancelling abilities only works with the ability key and nothing else.

Kelvin also cannot use his dome on himself even when using the use on self-ability key. He can only use it on teammates or enemies but never himself.

 

[Dartjuk]

Cancel button not working seems to be intentional on devs part, since you can cancel Kelvin's ult while using beam now. Imagine you are using beam inside your ult, and then you want to cancel ult, but not the beam. If you press space key to cancel it, instead of ability key, it will cancel both ult and beam. I think it's a temporary measure. They need to add ability-specific custom cancel button in the settings, or some kind of setting for cancelling order.
As for self-use, double-click for self-use setting seems to work.