Does the voltage meter fluctuate slightly towards higher voltage side for a moment, when we disconnect the electric supply?
What do you mean by "disconnect the electric supply"? Do you mean, if I had a volt meter connected to the wall and disconnected it, would I see a voltage spike? I can't see how.
When there are transformers in a circuit, sudden changes can lead to transient spikes due to self-inductance. I'm not sure if this can be seen when the power is switched off, but perhaps someone knowledgeable can comment.
Can it happen by electrons retuning back to their origional shells by decaying back to more than one shell by sudden cut-off of continious supply of energy--which may not be there in somewhat live condition due to continious supply of energy which might had kept the electrons always at some higher level than the normal level?
No, that's pure nonsense. There's nothing like electrons being out of their shells or falling back into these purported shells when electricity is cut off. The flow of electricity has nothing to do with electrons being put into higher atomic energy levels. It has to do with the fact that the energy levels at which the electrons in a metal normally reside, are spread throughout the metal: electrons in a metal are mobile, not confined to one atom.