See my post right above yours for where I get it from. You are looking at fluff. I'm quoting the RAW.
Counterspell has a somatic component only. It may or may not be a forceful gesticulation. It is probably not an intricate set of gestures. Nothing in the rules you've stated says that it will or won't interrupt the casting of another spell."Copying that spell into your spell book involves reproducing the basic form of the spell, then deciphering the unique system of notation used by the wizard who wrote it. You must practice the spell until you understand the sounds or gestures required, then transcribe it into your spell book using your own notation."
"VERBAL (V)
Most spells require the chanting of mystic words. The words themselves aren't the source of the spell's power; rather, the particular combination of sounds, with specific pitch and resonance, sets the threads of magic in motion."
"SOMATIC (S)
Spellcasting gestures might include a forceful gesticulation or an intricate set of gestures. If a spell requires a somatic component, the caster must have free use of at least one hand to perform these gestures."
These aren't variable things. You practice the spell until you understand the specific sounds and gestures required. If you vary those, you aren't casting the spell. The spell will be the same each time a particular wizard casts the spell. It's not going to be longer this time and shorter next time. It won't have these gestures this time and those other gestures next time.
If a table took the view that counterspelling while casting requires a second free hand, thus ensuring that the caser has free use of at lesat one hand to perform the gestures of their primary spell I think that would be quite reasonable. That's how Doctor Strange would normally do it.
I think it would be equally reasonable to allow doing it all with one hand, but requiring an INT (Arcana) check against an appropriate DC. I'm sure that Doctor Strange could do it with one hand in a pinch.