Your solution seems okay to me... maybe something like this:
| rex field=MyInconsistentDateTimeField "(?<d>[^//]*)/(?<m>[^//]*)/(?<y>[^\s]*)\s+(?<h>[^:]*):(?<min>[^:]*)(:(?<sec>.*))?"
| eval y = substr(y, len(y)-1,2)
| fillnull value="00" sec
| eval newTime=printf("%02d/%02d/%02d %02d:%02d:%02d", d,m, y,h,min,sec)
| eval parsed=strptime(newTime,"%d/%m/%y %l:%M:%S")
There may be a better way - but this I think this'd work to normalize the field.
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