Was "zwo" often or ever used before radio, or was it developed as a result of radio? Or has it just always been used around crowds and loud noises? And have other numbers ever had similar morphological changes?
It has nothing to do with radio.
Zwo is the female version (from Middle and Old High German zwō, zwā) for the numeral Zwei:
whereas zween is the male version (from Middle and Old High German zwēne)
Nowadays it isn't any longer in this use. At least zween is marked as archaic.
Zwo is used colloquial and when you want to differentiate acoustically from Drei.
Eins, Zwo, Drei
Eins, Zwei, Drei
In der zwoten Hälfte des 18. Jahrhunderts hatte zwo zwo Höhepunkte lt. Google ngram, also weit vor der Erfindung des Radios 1923 etwa startete der Rundfunk im dt. Reich lt. Wikipedia :
In the second half of the 18th century, "zwo" had its all time high, according to google ngram, and this was long before the invention of the radio around 1923 the Rundfunk im dt. Reich started, according to the German Wikipedia.