Well, if you really want a question type like this, then it should not be necessary to customise core code. It would be possible to do this with a self-contained question type plugin, a bit like qtype_multichoiceset or qtype_oumultireponse.
And these plugins tend not to require very much code, because they can reuse a lot of qtype_multichoice. For example https://github.com/ecampbell/moodle-qtype_multichoiceset/blob/master/question.php.
Probably, for what you want, your question type would just need to override the start_attempt method, to change the randomisation, so that whichever option was in last place stays in last place, but the other choices are shuffled. That, plus all the boilerplate code that a question type needs.