The attack and decay controllers can make the volume ramp up and down slower, but do not affect when that ramp starts.

The duration of a note is determined in real time by the note-on and note-off messages. It is not possible to increase the length of the pause between two notes with a controller; the note cannot be switched off before the note-off message is received (the synth cannot see into the future, it does not know when the messages arrives until it actually happens).

To make a note shorter, you must change the timing of the note-on or -off messages.

Thank you for your feedback. How decay works? What is the difference between release and decay? Or both means the same? I am unable to get the decay part working using CC#75. release works fine using CC#72 or even the attack (CC#73) works fine.
In case decay means full volume for short duration (say configurable 25-50% of the ON time of the note) followed by reduced volume or no volume for remaining period, I should be OK with that. Basically I need abrupt reduction in volume half way through the note. I am able to some how manage by playing the note for around 25% and then playing silence for 75%. But for me it is a tedious work of splitting the duration and adding silence. If there is a property that I can set, it would have been wonderful.

A synthesised sound envelope goes Attack, Decay, Sustain, Release, hence ADSR.
After maybe a rapid attack to a high level, there is perhaps a decrease in level before the sustained part of the envelope then, when the key is released, there follows the release to zero.
As Clemens posted, if you want to make a note shorter move the Note Off command backwards in time to be nearer the Note On.
Then you will also have made the gap between that note and the next one longer.

Some synthesizers use more complex envelope generation. That is (DAHDSR, for example): note on - delay (no sound, what you need indeed!) - attack (volume rises) - hold (volume remains at maximum) - decay (it decreases) - sustain (it stops at some level) - note off - release (volume ramps to zero). But there is no standard cc (maybe NRPN?) for delay, hold, &c additional phases adjusts, they are properties of the "program", of your instrument (be your instrument like this).

Yes, "delay" seems be usefull.

The difference between decay and release phases is that the "decay" appears anyway after "attack", and sound may be sustained after that (even at maximum, in this case decay parameter is meaningless) or may become silent (so no release phase be) - depend on "sustain" parameter, but the "release" appears only after note-off and allways go to silence.