Is "deacceleratingly" a valid word?

You are correct, you can use words deaccelerate or deacceleratingly.

You won't find it in standard dictionaries like Merriam-Webster or Oxford. But you can certainly use them.

But, yes, it is true, I seldom came across the words called "deaccelerate" or “deacceleratingly" in any of the reference books while in my under-graduation in physics.

Also, deaccelerate is not a mother of decelerate. Actually, the verb decelerate(1899) is a backformation of the noun deceleration (1894).

You can use prefix de- to make combining forms e.g. deaccelerate or deacceleratingly:

1. de- is added to a verb in order to change the meaning of the verb to its opposite.
2. de- is added to a noun in order to make it a verb referring to the removal of the thing described by the noun.

So, yes you can use deaccelerate or deacceleratingly, it is not grammatically wrong.

The OP’s, very understandable, mistake was to think of accelerate as the ‘father’ of the word in question. This led the OP to add de- to it, analogously to the way in which we, for example, add de- to alcoholise to get dealcoholise, the verb for the opposite action. It is, however, better to think of decelerate and accelerate as siblings. The a- at the beginning of accelerate is itself a prefix; both words are thus formed by adding different prefixes to the common root. Their ultimate ancestor is the Latin word celer, meaning fast, swift, rapid; to accelerate is thus to make something fast, to decelerate is to make it the opposite of fast, that is slow. While, there is no word celerate in English, there is celerity which belongs to the same family, and makes the root more clearly visible. The relationship between accelerate and decelerate is analogous to that between appreciate and depreciate (note that we do not say deappreciate), not to that between alcoholise and dealcoholise.

Another way to think of it is this. What does the action of decelerating take away? It does not just take away acceleration: if one accelerates, and then merely takes away the acceleration, the result is that the motion continues at the speed, possibly very high, which was reached by the acceleration. The action of decelerating takes away the speed, velocity, celerity. That’s why the word is decelerate rather than deaccelerate.

Using the word deaccelerate is thus not merely a stylistic faux pas that would annoy some stuffy linguistic prescriptivists, but is otherwise harmless. It can create confusion, because when we hear it, we cannot be sure whether it is used as a variant of decelerate, or to mean removing the accelaration (but possibly continuing at high speed), which is what it would mean, according to the principles that generally govern the prefix de-.