Should Brendan Horan stay in Parliament?

Having read the allegations and counter-allegations coming from Brendan Horan's family, I figure that it's best not to comment directly on them.
But, the fact is that he has been expelled from the NZ First caucus and will, for the time being, sit as an independent MP. So, this raises one of the issues with MMP: Should a list MP who has been expelled from caucus automatically be expelled from Parliament too? Many of us might jump to the conclusion that, by law, someone in Horan's position should not be permitted to stay on as an MP at all. But I suggest that we first look at both sides of the argument.
First, I'll argue the case against automatic expulsion from Parliament.
If such a law existed, there would be much more risk involved for a list MP if he/she wished to challenge an unpopular party policy or to expose wrongdoing within the party. Accusations could easily be trumped up against a renegade MP in order to justify expulsion from caucus, and hence from the House altogether. It's easy to imagine a situation where this could have consequences that the public might find undesirable. In Horan's case, he is still maintaining that the allegations against him are false, and so automatic expulsion from the House would seem to be unfair, unless and until the allegations were to be proven. If he were to resign voluntarily as an MP even, that could be interpreted as an admission of guilt, even though he has a right to be considered innocent until proven guilty.
Furthermore, before the last election, the NZ First party list, as published, did have his name on it. Hence, it could be said that those who voted to put NZ First into Parliament were in effect voting partly for Brendan Horan too. So, to argue that he has no personal mandate to sit in the House is not quite correct.
So, let me turn now to the counter-argument: that someone in Horan's position should have to resign (or be sacked) as an MP. The people who voted for a party gave that party a mandate as a whole, and they expect their party to be proportionally represented in the House. The loss of one of their list MPs (by voluntary resignation or by expulsion) then reduces that party's proportionality. Hence, the party should be able to call on the next person in line on their party list to occupy the former member's seat. No-one wants a lame-duck independent MP sitting on an expensive salary achieving very little. Would it not be better to see the former party representative expelled from the House and replaced by a more effective member who can represent those who voted for that party?
Even though there is presently no law to force such an independent MP from the House, we might argue that there is at least a moral obligation for an MP in Horan's position to leave. But, we could easily also say that there is a moral obligation for him to stay, so long as the allegations are unproven, and in order to discourage party leaders from bullying their list MPs into obedience.
If, on the other hand, the allegations against him were to be proven, then that would lend strength to the argument that he does not deserve his seat in Parliament.