Partial answer, not seeing how they all fit together...
I am an involuntary action
I will make you cry
If you force me to stop
Blinking, an involuntary action, trying not to blink will make your eyes water.
I am also a knave
and of luck
The Jack of Spades in a deck of cards. Jacks are also called "Knaves". A spade is used for excavation, or digging. And various cards represent luck - the ace of spades is regarded as lucky.
I am also a soldier
you can't find me in the heavens
you can't find me in the seas
Not 100% sure but I think this is talking about infantry, soldiers on the ground (as opposed to navy on the seas and airforce in the skies). Maybe you are looking for another synonym of infantry.
In a lifetime
and upon a time
I am what's missing
Once is the missing word from the idioms "once in a lifetime" and "once upon a time".