My money is on cock-up rather than conspiracy.
The SNP haven't had a leadership election for decades and the last time their membership was two men and a dog. Also I don't think anyone was really expecting an election now rather than in a few years, so they will have been caught on the backfoot.
I suspect they've also woken up to the fact that although they've been used to doing stuff with decent resources for admin in government, this vote cannot use government resources, only SNP party resources which will be limited. I've also noted that other parties have had problems identifying how many members they have and who can vote. This is exacerbated with parties that rely greatly on constituency organisations where national level lists are a collation of local lists. Not sure if the SNP works in this way, but a brief glance at their rules suggest it might.
The vote may be an almighty mess due to incompetence but that isn't the same as it being rigged - to rig an election, I would suggest, actually requires competence and you wouldn't do it by stopping some legitimate members getting to vote - why? Because those people not receiving ballot papers will go straight to the press and also you'd need to know which way those who get ballot papers, and those that do not, will vote - I don't think that is possible to know reliably.
But then right from the start of this thread I've criticised the speed of the process - and this is one of the consequences. - pretty difficult to put in place a process within a few weeks from a standing start and when you haven't done anything similar for decades, realistically if at all.
That said - sure they should have independent oversight of the process - whenever I've voted in Union elections or (I think) in Labour party elections this has been run through an independent organisation, often the Electoral Reform Society.
What we have here is a false dichotomy -caused by thinking that politicians's are saying what they mean, and by cincentrating on this being all about the election process.
I doubt that Forbes or the vast majority of her supporters think there is a widespread manipulation of the votes. Rather is playing into trying to pick up the second votes from Regan supporters by appearing as definitely not a continuity candidate - there's been some mumblings amongst the more imaginative of Regan's supporters that the attacks by Yousaf and Forbes on each other are all a front and that both are chosen continuity candidates.
Even Regan's supporters, well the ones in the SNP, aren't really arguing that the votes being highly manipulated. There are ones outside the SNP who think that Yousaf is a CIA plant.
The overall issue is much more to do with the whole problem of having had a married couple who were leader and CEO. I've always been amazed that more wasn't made of this by the other parties as Sturgeon's husband is Caesar's wife.
You then overlay that with the various previous problems, the Dalmond case and who knew what when, the alphabetties in the Salmond case, the 'missing' £600,000 and the police investigations into that, the £107,000 loan from Murrell that Sturgeon was apparently unaware of when it was made, an ongoing question about membership numbers, resignations from the finance committee due to not being able to see the finances.
The relatively short election process is then seen as at least an attempt to reduce the possibility of such things being raised.
Your point about independent oversight is interesting as the argument from the SNP central team is that it is done independently by MiVoice. The challenge to them has been that MiVoice do not appear to be validating the electorate at all. It's not clear what the situation is - which then links bacl to the lack of transparency in the election being seen as a symptom of the overall lack of transparency.