On whether the side effects of Cheque Drops “should” be due to a factor other than the steroid itself, I would not assume this.
Side effects of androgens, particularly psychological effect and things like nausea and headaches, are completely unpredictable by the human mind from structure. (It may be that there are by now computer programs that can predict binding to all receptors that could be relevant for such things – a number of years back it was possible to predict from structure binding to androgen, estrogen, and corticosteroid receptors, and more types may have been added since – but I wouldn’t be surprised if that still is not fully possible.)
It’s completely ordinary for a small structural change to result in completely different side effects from a steroid.
For example, how could it have been predicted that boldenone – “same as Dianabol but without the 17a-methyl” – could cause fever or fevered feeling at higher doses? How could it have been predicted that methyl-1-T would have a worse psychological side effect profile than 1-T? For that matter, how could it have been predicted that 1-T had allopregnanolone-type activity that could cause depression in users? Etc.
And not only is it unpredictable as to whether mibolerone may inherently have the side effects that Cheque Drops demonstrate, there’s also the fact that the patent holder decided never to bring the drug to market for humans. Why give up that profit? That doesn’t prove anything but is suggestive.
The impurity idea would be very plausible if talking about some underground thing, but – don’t know about presently but for very many years during which time Cheque Drops earned their reputation – they were made with pharmaceutically-manufactured mibolerone with the only thing added, I believe, being propylene glycol if I recall correctly. (And maybe a B vitamin? Not sure.) So that leaves it pretty unlikely that anything but the mibolerone itself was to blame, in this instance, though the general suggestion certainly remains valid for exotic underground steroids which, if they do odd things, might indeed have impurities as the cause as you suggest.