I think you may be on the right track by breaking down the values.
| rex field=MyInconsistentDateTimeField "^(?[\d]+)\/.*"
| rex field=MyInconsistentDateTimeField "^[\d]+\/(?[\d]+)\/.*"
| rex field=MyInconsistentDateTimeField "^[\d]+\/[\d]+\/(?[\d]+)\/.*"
| rex field=MyInconsistentDateTimeField ".(?[\d]+):."
| rex field=MyInconsistentDateTimeField ".:(?[\d]+)."
| fields MyInconsistentDateTimeField, day1, month1, year1, hour1, minute1
check that and see if it grabbed the right values, from that you can build on it as you need.
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